Tag Archives: java script

Simple use of Promises in JavaScript

Here is a simple and a bit silly example of using promises. In real life we could just move the logic into done and fail functions which are promises themselves but for the sake of explaining how promises work we are going to do this example.

you could see the example here.

 var promise = new Promise(function(resolveFunc, rejectFunc) {

    $.ajax({
      url: "https://www.toni-develops.com/external-files/examples/service-workers/delayed-response.php",
    })
      .done(function( data ) {
        resolveFunc(data);
      })
      .fail(function(error) {
          rejectFunc(error);
      });

});

promise.then(
    function(result) { // resolve func
        console.log('Success! Data: ', result);
}, function(error) {    // reject function
    console.log("Error! ", error);
})

console.log("Promise was called, and once it's fulfilled the code inside the 'then' function will be executed!");

What’s happening here:
– Line 1 creates a new promise, which does AJAX request and calls done or fail functions.
– If execution is successful and done function is called, it also invokes resolveFunc passing the result data.
– Line15 is where the promise has been called, and the bodies of resolveFunc (line 16) and  rejectFunc (line 18) are defined and passed to the promise.

Run the code and you should see first the message “Promise was called, and once it’s fulfilled the code inside the ‘then’ function will be executed!
Five seconds later the newly fetched content will be shown since the service intentionally takes 5 seconds to return the data for testing purposes.

What if we want to make it much easier? Let’s use async and await

async function fetchData() {
    var result = await fetch('https://www.toni-develops.com/external-files/examples/service-workers/delayed-response.php');
    return result;
}


(async () => {
    var r = await fetchData();
    console.log("ONE::", r);    
    console.log("DONE !");
})();

 

Maximum Subarray

Task

Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum.

Example:

Input:

 [-2,1,-3,4,-1,2,1,-5,4],

Output:

 6

Explanation:

 [4,-1,2,1] has the largest sum = 6.

This problem was taken from Leetcode

Solution

The solution:

Brut force.

The straight forward solution could be to iterate through all elements in the array and calculate the subarray values and compare them.

In the example above[-2,1,-3,4,-1,2,1,-5,4] we will do:

starting index: 1
-2 = -2
-2, 1 = -1
-2, 1, -3 = -4
-2, 1, -3, 4 = 0
-2, 1, -3, 4, -1 = -1
-2, 1, -3, 4, -1, 2 = 1
-2, 1, -3, 4, -1, 2, 1 = 2
-2, 1, -3, 4, -1, 2, 1, -5 = -3
-2, 1, -3, 4, -1, 2, 1, -5, 4 = 1

starting index: 2
1 = 1
1, -3 = -2
1, -3, 4 = 2
1, -3, 4, -1 = 1
1, -3, 4, -1, 2 = 3
1, -3, 4, -1, 2, 1 = 4
1, -3, 4, -1, 2, 1, -5 = -1
1, -3, 4, -1, 2, 1, -5, 4 = 3

starting index: 3
-3 = -3
-3, 4 = 1
-3, 4, -1 = 0
-3, 4, -1, 2 = 2
-3, 4, -1, 2, 1 = 3
-3, 4, -1, 2, 1, -5 = -2
-3, 4, -1, 2, 1, -5, 4 = 2

starting index: 4
4 = 4
4, -1 = 3
4, -1, 2 = 5
4, -1, 2, 1 = 6
4, -1, 2, 1, -5 = 1
4, -1, 2, 1, -5, 4 = 5

… and so on till the last element in the array.

So the winner clearly is 4, -1, 2, 1 = 6, but this approach will take a lot of repetitions. Interestingly there is a linear solution called: Kadane’s algorithm.

Using Kadene’s algorithm.

The basic idea of this algorithm is to break the array into a sets of mutually exclusive sets, calculate their sums and find the largest one.

First let’s look closely of what we are doing to find the maximum sum using brut force. We are splitting the array to a sets of all possible contiguous sub arrays and we calculate their sum. This means that:
– if the array contains only negative values we don’t really need to split the array cause the answer will be the largest value in the array. i.e. [-1,-5,-3] = -1 (the one close to 0)
– if this is a mixed array with negative and positive values the max sum of contiguous sub array will be > 0 so we could ignore any case where the sum is negative.

This way we could iterate through each element of the array nums[i]  where i is the index of the element in the array (starting with the first one nums[0]), and calculate the sum (let’s call it max_here = max_here + nums[i] ).
If we get a negative result we already know for sure that this is not what we are looking for and we set up max_here to the next element in the array max_here = nums[i]

So in the example above: [-2,1,-3,4,-1,2,1,-5,4]
We are starting by setting up both params to the first element in the array: max_here =max_so_far = nums[0] . We are going to use max_so_far to store the maximum sum discovered so far, and max_here to calculate the maximum sum so far. Once again if the max_sum is negative, we just set it up to be equal to the next element in the array nums[i] so on the next iteration max_sum = nums[i-1] + nums[i]

Starting with setting up max_here = max_so_far = nums[0] = -2

i nums[i]    action described max_here max_so_far
1 1  sum = max_here + nums[1], which is:
sum = -2 + 1 = -1 which is smaller than nums[1] so max_here = nums[1] = 1 (line 10 in the code snipped below)
and since max_here > max_so_far,  max_so_far = max_here =1 (line 11)
1 1
2 -3  sum = max_here + nums[2] which is:
sum = 1 + (-3) = - 2 which is bigger than nums[2] which is -3 so max_here = sum = -2But max_here is smaller than max_so_far so max_so_far stays equal to 1
-2 1
3 4  sum = -2 + 4 = 2 which is < than 4 so  max_here = nums[4] = 4 which is > max_so_far so max_so_far = max_here = 4 4 4
4 -1   sum = 4 - 1 = 3 > – 1 so
max_here = sum = 3
3 4
5 2  sum = 3 + 2 = 5
which is > than nums[5] = 2 so
max_here = max_so_far = sum = 5
5 5
6 1  sum = 5 + 1 = 6
which is > than nums[6] = 1 so
max_here = max_so_far = sum = 6
6 6
7 -5  sum = 6 + (-5) = 1
which is > than nums[7] = -5 so
max_here = 1
1 6
8 4  sum = 1 + 4 = 5
which is > than nums[8] = 4 so
max_here = 5 but
max_here < max_so_far so
max_so_far stays the same: 6 which is the maximum sum here.
5 6

 

/**
 * @param {number[]} nums
 * @return {number}
 */
var maxSubArray = function(nums) {
    var max_here = max_so_far = nums[0];
    

    for(var i=1;i < nums.length; i ++) {
        max_here = Math.max(max_here + nums[i], nums[i]);
        max_so_far = Math.max(max_so_far, max_here);
    }    

    return max_so_far;
};

 

 

Adding environment variables file.

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Although Wepack already comes with two modes: development and production, It comes handy to have different environment variables for development and production and to store them in different files.

Let’s create .env file which will be our development environment variable file.

Creating the .env file

./.env

APP_NAME=Webpack React Tutorial
GRAPHQL_URL=http://localhost:4001/graphql

As it looks like .env is not a standart JavaScriupt object so we can’t use the variables out of the box. We will need two modules Dotenv and Dotenv-expand.

Adding and configuring Dotenv to load variables into process.env.

Dotenv is module that loads environment variables from a .env file into process.env.

yarn add dotenv dotenv-expand

Let’s give it a try and print out GRAPHQL_URL in the backend. Let’s do this using server-api.js config. All that we need to do is to ‘tell’ Dotenv module to load variables into process.env

./src/server-api.js

import WebpackDevServer from 'webpack-dev-server';
import webpack from 'webpack';
import config from './webpack.api.config.js';
require('dotenv').config();

console.log(">>>" + process.env.GRAPHQL_URL);

const compiler = webpack(config);
const server = new WebpackDevServer(compiler, {
  hot: true,
  publicPath: config.output.publicPath,
  historyApiFallback: true
});
server.listen(8080, 'localhost', function() {});

Run the project and in the backend console you will see GraphQL url printed out.

Now, let’s create getEnvironmentConstants helper method that will use Dotenv to load variables, and in addition we will add a filter that will load only these variables to the front end that we specify in frontendConstants . This way important variables that we need in the backend like passwords to the database won’t be exposed in the source code.

./getEnvironmentConstants.js

const fs = require('fs');

// Load environment variables from these files
const dotenvFiles = [
  '.env'
];


// expose environment variables to the frontend
const frontendConstants = [
  'APP_NAME',
  'GRAPHQL_URL'
];

function getEnvironmentConstants() {
  
  dotenvFiles.forEach(dotenvFile => {
    if (fs.existsSync(dotenvFile)) {
      require('dotenv-expand')(
        require('dotenv').config({
          path: dotenvFile,
        })
      );
    }
  });
  
  const arrayToObject = (array) =>
  array.reduce((obj, item, key) => {
    obj[item] = JSON.stringify(process.env[item]);
    return obj
  }, {})

  return arrayToObject(frontendConstants);      
}

module.exports = getEnvironmentConstants;

Once we have the object in place we could use the DefinePlugin to pass them to the frontend.

Adding the DefinePlugin.

if we go back in this tutorial we will remember that we showed how to configure Webpack in three different ways: using CLI, the webpack API and the server middleware.
Now the best place to add DefinePlugin will be in webpack.base.config so all three Webpack set-ups will take advantage of it. Let’s import getEnvironmentConstants and pass it as a parameter to DefinePlugin.

./webpack.base.config.js

import getEnvironmentConstants from './getEnvironmentConstants';
import webpack from 'webpack';

module.exports = {
  mode: 'development',
  devtool: 'eval-source-map',
  entry: [
    '@babel/polyfill',    
    './src/index.js',
  ],
  output: {
    filename: '[name]-bundle.js',
    publicPath: '/dist',
  },  
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: "babel-loader"
        }
      },

      // SCSS
      {
        test: /\.scss$/,
        use: [
          'style-loader',
          {
            loader: 'css-loader',
            options: {
              modules: true,
              importLoaders: 2,
              localIdentName: '[folder]-[local]',
              sourceMap: true
            }
          },
          {
            loader: 'postcss-loader',
            options: {
              plugins: () => [require('autoprefixer')()],
              sourceMap: true
            },
          },
          {
            loader: 'sass-loader',
            options: {
              outputStyle: 'expanded',
              sourceMap: true
            }
          }
        ],
      },
      // images
      {
        test: /\.(png|jp(e*)g|svg)$/,  
        use: [{
            loader: 'url-loader',
            options: { 
                limit: 8000, // Convert images < 8kb to base64 strings
                name: 'images/[hash]-[name].[ext]'
            } 
        }]
      },
      //File loader used to load fonts
      {
        test: /\.(woff|woff2|eot|ttf|otf)$/,
        use: ['file-loader']
      }                    
    ]
  },
  plugins: [
    new webpack.DefinePlugin({ 'process.env' : getEnvironmentConstants() } )
  ]
};

 what we just did:
– (line 1 and 2) we imported getEnvironmentConstants and Webpack since we will need it to instantiate the plug in.
– (line 72-74) we added the plug in.

We have to do one more change in order to have the plug-in working for all Webpack configs:

./webpack.api.config.js

const webpack = require('webpack');
let config = require('./webpack.base.config.js');

config.entry = [
  '@babel/polyfill',    
  './src/index.js',
  'webpack/hot/dev-server',
  'webpack-dev-server/client?http://localhost:8080/',       
];

config.plugins = [... [new webpack.HotModuleReplacementPlugin()], ... config.plugins ];

module.exports = config;

and

./webpack.middleware.config

const webpack = require('webpack');
let config = require('./webpack.base.config.js');

config.entry = [
  '@babel/polyfill',    
  './src/index.js',
  'webpack-hot-middleware/client?path=/__webpack_hmr&timeout=20000',        
];

config.plugins = [... [new webpack.HotModuleReplacementPlugin()], ... config.plugins ];

module.exports = config;

 what we just did:
Since we added config.plugins array in webpack.base.config we don’t want to override it here and lose the changes. That’s why we are merging the array using spread operator.
If you are unfamiliar with the spread operator you might read the link above. Basically what it does is to ‘spread’ two or more arrays into the current array.

var a = [1,2,3];

var b = [...a, ...[4,5,6]];

console.log(b);

result: (6) [1, 2, 3, 4, 5, 6]

Accessing env variables in the front-end.

And let’s load these variables. We could replace the hardcoded GraphQL url with the one from the .env file.

And using the variables on the back end is straight forward: we just include .env file and use the variables, but passing them to the front end requires a little bit more effort. We have to use the DefinePlugin which will allow us to create global constants which can be configured at compile time.

./src/components/App/index.js

import React, { Component } from 'react';
import PageLayout from '../../containers/PageLayout';
import { ApolloProvider } from 'react-apollo';
import { ApolloClient } from 'apollo-client';
import { HttpLink } from 'apollo-link-http';
import { InMemoryCache } from 'apollo-cache-inmemory';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
import { Provider } from 'react-redux';
import { createStore} from 'redux';
import reducers from '../../reducers';

import styles from './styles.scss';

const store = createStore(reducers, {});
export default class App extends Component {
  render() {
    const GRAPHQL_URL = process.env.GRAPHQL_URL;
    const client = new ApolloClient({
      link: new HttpLink({ uri:  GRAPHQL_URL }),
      cache: new InMemoryCache()
    });  
    return (
      <div className={styles.appWrapper}>
        <Provider store={store}>
          <ApolloProvider client={client}>
            <Router>
              <Switch>
              <Route exact path="*" component={PageLayout} />  
              </Switch>
            </Router>
          </ApolloProvider>
        </Provider>
      </div>        
    );
  }
}

 

And we could also print the APP name in the header section process.env.APP_NAME

./src/components/Header/index.js

import React from 'react';
import { Link } from 'react-router-dom';
const styles = require('./styles.scss');
const Header = ( {title} ) => (
  <div>
    <div className={styles.wrapper}>      
      <h2>{ title } { process.env.APP_NAME } </h2>
      <ul>
        <li><Link to='/home'>HOME</Link></li>
        <li><Link to='/greetings'>GREETINGS</Link></li>       
        <li><Link to='/dogs-catalog'>DOGS CATALOG</Link></li>
        <li><Link to='/about'>ABOUT</Link></li>
      </ul>
    </div>
  </div>
);
export default Header;

Now, start the server using yarn start-api and if everything works fine you will see the “Webpack React Tutorial” in the header.

 

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Breaking the schema into separate group of files.

Having the entire schema into the server.js file is not practical, especially if the schema grows bigger, so let’s move the schema into separate folders for easy management:

mkdir ./src/schema/queries -p
mkdir ./src/schema/types -p
mkdir ./src/schema/mutations -p

./src/schema/index.js

const graphql = require('graphql');
const queries = require('./queries');
const mutations = require('./mutations');

var rootQuery = new graphql.GraphQLObjectType({
  name: 'Query',
  fields: {...queries, ...mutations },
});

module.exports = new graphql.GraphQLSchema({
  query: rootQuery
});

./src/schema/types/index.js

let Dogs = require('./dogs.js');

module.exports = {
  ...Dogs,
};

./src/schema/types/dogs.js

const graphql = require('graphql');

module.exports = new graphql.GraphQLObjectType({
  name: 'dogType',
  fields: {
    id: { type: graphql.GraphQLString },
    breed: { type: graphql.GraphQLString },
    displayImage: { type: graphql.GraphQLString },
  }
});

./src/schema/queries/index.js

let Dogs = require('./dogs.js');

module.exports = {
  ...Dogs
};

./src/schema/queries/dogs.js

const graphql = require('graphql');
const dogType = require('../types/dogs');
const dogs = require('../../models/mock_data/dogs.js');

module.exports = {
  getDogByBreed: {
    type: dogType,
    args: {
      breed: { type: graphql.GraphQLString }
    },
    resolve: function (_, {breed}) {
      var result = dogs.find(function(dog){
        return breed == dog.breed;
      });
      return result;
    }
  } 
}

./src/schema/mutations/index.js

let Dogs = require('./dogs.js');

module.exports = {
  ...Dogs
};

./src/schema/mutations/dogs.js

const graphql = require('graphql');
const dogs = require('../../models/mock_data/dogs.js');

module.exports = {
  addDogBreed: {
    type: graphql.GraphQLString,
    args: {
      id: { type: graphql.GraphQLString },
      breed: { type: graphql.GraphQLString },
      displayImage: { type: graphql.GraphQLString }
    },
    resolve: function (_, {id, breed, displayImage}) {
      dogs.push({
        id: id,
        breed: breed,
        displayImage: displayImage
      });
      return "OK!";
    }
  }   
}

./server.js

var express = require('express');
var graphqlHTTP = require('express-graphql');
const schema = require('./src/schema');


// Logger middleware
var logger = function(req, res, next) {
  console.log("GOT REQUEST >", req.ip);
  next(); // Passing the request to the next handler in the stack.
}

var app = express();
app.use(logger);
app.use('/graphql', graphqlHTTP({
  schema: schema,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at localhost:4000/graphql');

 

Setting up faster development environment using Nodemon and Babel.

Adding Nodemon

Nodemon is the same like node with the benefit that it will monitor the developing folder and restart automatically,, which makes the developing process more convenient. Let’s go ahead and install it:

yarn add nodemon --dev

And replace the node with nodemon in the start script:

./package.json

{
  "name": "graphql-tutorial",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "scripts": {
    "start": "nodemon server.js"
  },
  "dependencies": {
    "express": "^4.16.4",
    "express-graphql": "^0.7.1",
    "graphql": "^14.0.2"
  },
  "devDependencies": {
    "nodemon": "^1.18.9"
  }
}

Now, whenever we edit and save a file Nodemon will conveniently restart for us and reflect the new changes.

If we also want to get advantage of the latest ES syntax we would like to install Babel.

 

Adding Babel 7

Starting with Babel7 the config setting changes quite a bit.

  1. Babels packages are now scoped and Babel has renamed it’s NPM packages. This means babel-cli for example has been renamed to @babel/cli .
  2. No messing around with presets anymore. You can just use @babel/preset-env now and optionally define your requirements in the config file.
  3. babel-node has been moved from the CLI to it’s own package: @babel/node

Let’s go ahead and install all necessary packages:

yarn add @babel/core @babel/cli @babel/node --dev

and tell nodemon to use Babel to transpile JS.

./package.json

{
  "name": "graphql-tutorial",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "scripts": {
    "start": "nodemon --inspect=10111 --exec babel-node server.js"
  },
  "dependencies": {
    "express": "^4.16.4",
    "express-graphql": "^0.7.1",
    "graphql": "^14.0.2"
  },
  "devDependencies": {
    "nodemon": "^1.18.9"
  }
}

 

Before we could start using the new ES features we have to install a preset and tell Babel to use it by adding ./babelrc config file

yarn add @babel/preset-env --dev

add

./.babelrc

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "useBuiltIns": "entry"
      }
    ]
  ]
}

 

give it a try …

./server.js

import express from 'express';
import graphqlHTTP from 'express-graphql';
import schema from './src/schema';


// Logger middleware
var logger = function(req, res, next) {
  console.log("GOT REQUEST >", req.ip);
  next(); // Passing the request to the next handler in the stack.
}

var app = express();
app.use(logger);
app.use('/graphql', graphqlHTTP({
  schema: schema,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at localhost:4000/graphql');

 

Using GraphQLSchema to construct the schema programmatically.

For most of the cases defining a fixed schema when the application starts, by adding Query and Mutation types solely using schema language is good enough. But sometimes we might need to define a dynamic schema and we can achieve this by creating a new JS objects.

Construct dynamic schema for ‘User Greeting’ example.

Let’s get again to the ‘Greetings user’ example, because of it’s simplicity and define a Query with field named greetingUser which will accept userName and bornMonth parameters, first of type string and the second of type int and return userType.

And the userType will return greetingOne  which will simply say “Hello [userName]” and greetingTwo  that will let the user know how many months left till their next birthday. Both of type string.

./src/server.js

var express = require('express');
var graphqlHTTP = require('express-graphql');
var { buildSchema } = require('graphql');
var dogs = require('./src/models/mock_data/dogs.js');
const graphql = require('graphql');

// Define the User type
var userType = new graphql.GraphQLObjectType({
  name: 'UserType',
  fields: {
    greetingOne: { type: graphql.GraphQLString },
    greetingTwo: { type: graphql.GraphQLString },
  }
});

// Define the Query type
var queryType = new graphql.GraphQLObjectType({
  name: 'Query',
  fields: {
    greetingUser: {
      type: userType,
      args: {
        userName: { type: graphql.GraphQLString },
        bornMonth: { type: graphql.GraphQLInt }
      },
      resolve: function (_, {userName, bornMonth}) {
        var date = new Date();
        var daysLeft = bornMonth - (date.getMonth() + 1);
        daysLeft = daysLeft < 0 ? daysLeft + 12 : daysLeft;
        return {
          greetingOne: `Hello ${userName}`,
          greetingTwo: `Your birthday is comming in ${daysLeft} month(s)`
        };
      }
    }
  }
});

var schema = new graphql.GraphQLSchema({query: queryType});


// Logger middleware
var logger = function(req, res, next) {
  console.log("GOT REQUEST >", req.ip);
  next(); // Passing the request to the next handler in the stack.
}

var app = express();
app.use(logger);
app.use('/graphql', graphqlHTTP({
  schema: schema,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at localhost:4000/graphql');

what we just did:
– we defined the user type which is pretty self explanatory (Lines 8-14)
– then we created the query type, that has these parameters:
type which is the return type, in this case userType
args is the input parameter types.
resolve is the resolver function.

Transform Dogs catalog to use dynamic schema.

Creating the dog type

var dogType = new graphql.GraphQLObjectType({
  name: 'dogType',
  fields: {
    id: { type: graphql.GraphQLString },
    breed: { type: graphql.GraphQLString },
    displayImage: { type: graphql.GraphQLString },
  }
});

Creating the query type

// Define the Query type
var queryType = new graphql.GraphQLObjectType({
  name: 'Query',
  fields: {
    getDogByBreed: {
      type: dogType,
      args: {
        breed: { type: graphql.GraphQLString }
      },
      resolve: function (_, {breed}) {

        var result = dogs.find(function(dog){
          return breed == dog.breed;
        });
        return result;
      }
    }
  }
});

Creating a mutation type

...
    addDogBreed: {
      type: graphql.GraphQLString,
      args: {
        id: { type: graphql.GraphQLString },
        breed: { type: graphql.GraphQLString },
        displayImage: { type: graphql.GraphQLString }
      },
      resolve: function (_, {id, breed, displayImage}) {
        dogs.push({
          id: id,
          breed: breed,
          displayImage: displayImage
        });
        return "OK!";
      }
    } 
...

Adding the query schema

...
var schema = new graphql.GraphQLSchema({query: queryType});

...
app.use('/graphql', graphqlHTTP({
  schema: schema,
  graphiql: true,
}));
...

Putting it all together

var express = require('express');
var graphqlHTTP = require('express-graphql');
var { buildSchema } = require('graphql');
var dogs = require('./src/models/mock_data/dogs.js');
const graphql = require('graphql');

// Define the dogs type
var dogType = new graphql.GraphQLObjectType({
  name: 'dogType',
  fields: {
    id: { type: graphql.GraphQLString },
    breed: { type: graphql.GraphQLString },
    displayImage: { type: graphql.GraphQLString },
  }
});

// Define the Query type
var queryType = new graphql.GraphQLObjectType({
  name: 'Query',
  fields: {
    getDogByBreed: {
      type: dogType,
      args: {
        breed: { type: graphql.GraphQLString }
      },
      resolve: function (_, {breed}) {

        var result = dogs.find(function(dog){
          return breed == dog.breed;
        });
        return result;
      }
    },
    addDogBreed: {
      type: graphql.GraphQLString,
      args: {
        id: { type: graphql.GraphQLString },
        breed: { type: graphql.GraphQLString },
        displayImage: { type: graphql.GraphQLString }
      },
      resolve: function (_, {id, breed, displayImage}) {
        dogs.push({
          id: id,
          breed: breed,
          displayImage: displayImage
        });
        return "OK!";
      }
    }    
  }
});


var schema = new graphql.GraphQLSchema({query: queryType});


// Logger middleware
var logger = function(req, res, next) {
  console.log("GOT REQUEST >", req.ip);
  next(); // Passing the request to the next handler in the stack.
}

var app = express();
app.use(logger);
app.use('/graphql', graphqlHTTP({
  schema: schema,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at localhost:4000/graphql');

Plus One

Task

Given a non-empty array of digits representing a non-negative integer, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit.

You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:

Input:

 [1,2,3]

Output:

 [1,2,4]

Explanation:

 The array represents the integer 123.

Example 2:

Input:

 [4,3,2,1]

Output:

 [4,3,2,2]

Explanation:

 The array represents the integer 4321.

This problem was taken from Leetcode

Solution

The solution:

The solution is pretty straight forward. We traverse all digits in the opposite direction, and make sure that we add +1 only on the last digit (the first iteration ). Then if the digit > 9 we set up the dit to 0, and we carry on 1 to add it to the next digit, and keep going till we reach the first digit. If the first digit is 9 and cary over is not 0, we add 1 to the beginning of the array.

let’s consider: 9 9 9

iteration 1:
9            9
9            9
9 + 1 =  0 + cary on: 1

iteration 2:
9            9
9 + 1 =  0 + cary on: 1
0            0

iteration 3:
9 + 1 = 0 + cary on: 1
0          0
0          0

finally:
1     << adding leading 1 
0
0
0

which gave up the final result: 1 0 0 0

The solution will look like this:

Java Script

/**
 * @param {number[]} digits
 * @return {number[]}
 */
var plusOne = function(digits) {
    
    var carryOn = 0;
    for(q = digits.length - 1; q!=-1; q--) {
        var digit = digits[q];
        if(q == digits.length - 1) {
            digit = digit + 1;
            if(digit == 10) {
                digit = 0;
                carryOn = 1;
            }
        }
        else {
            if(digit == 9 && carryOn) {
                digit = 0;
            }
            else {
                digit = digit + carryOn;
                carryOn = 0;
            }
        }
        digits[q] = digit;
    }
    if(carryOn > 0) 
        digits.unshift(1);
    return digits;
};

 

what we just did:
– (lines 11 – 18) happened only if this is the last digit, where we have to add 1
– (lines 18 – 20) if we have carry on value of carryOn > 0 and digit = 9 simply set digit = 0 and left carryOn to be equal to 1 for the next iteration.

Valid Parentheses

Task

Given a string containing just the characters '('')''{''}''[' and ']', determine if the input string is valid.

An input string is valid if:

  1. Open brackets must be closed by the same type of brackets.
  2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input:

 "()"

Output:

 true

Example 2:

Input:

 "()[]{}"

Output:

 true

Example 3:

Input:

 "(]"

Output:

 false

Example 4:

Input:

 "([)]"

Output:

 false

Example 5:

Input:

 "{[]}"

Output:

 true

This problem was taken from Leetcode

Solution

The solution:

Let’s look at the simplest scenario where we have only one type of brackets: ‘(‘ and ‘)’. Then all that we need to do in order to figure out if each bracket has corresponding closing bracket is to put each opening bracket into a stack, and pop one bracket when we see closing bracket.

Ideally if the brackets are “normalized” (all of the open one have corresponding closing brackets) we will end up with empty stack.

in example :

( ( ) ( ) ( ( ) ) )
1 2  3  4 5  6  7  8  9  10

so here are all 10 steps:

steps  stack
1          (
2          (  (
3         (
4         (  (
5         (
6         (  (
7         (  (  (
8         (  (
9         (
10

Immediately it becomes clear that if the string  length is not an even number, it automatically becomes invalid. So we could do this check in the very beginning (lines 9 and 10)  below.

So let’s look at the current example where we have 3 different tags: ‘{}’, ‘()’, ‘[]’

The rule for a valid string is:
open tags:
– we could have as many open tag as we want. i.e. : ({[[ ...
closing tags:
– every closing tag should match previously opened tag ([]) – valid, ([)] – invalid.

So now we know the rules, let’s write the code.

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function(s) {
    
    var input = s.split('');
    
    if(input.length % 2 != 0)
        return false;
    
    var stack = [];

    var tagIndex = {
        '(': 0,
        ')': 1,
        '{': 2,
        '}': 3,
        '[': 4,
        ']': 5
    };        


    for(var q=0;q < input.length; q++) {
        var symbol = input[q];
        var tagType = (tagIndex[symbol] % 2); // 0 - open, 1 - close
        if(tagType == 0) {
            stack.push(symbol);
        }
        else {
            // this is a closing tag, make sure that it follows the rules
            lastTag = stack.pop();            
            lastTagIndex = tagIndex[lastTag];
            if( tagIndex[symbol] != lastTagIndex + 1 )
                return false;
        }
    }
    
    if(stack.length > 0)
        return false;
    return true;
};

 

what we just did:
– lines 9 and 10: we check if the length of the string is odd and return false immediately if so.
– we added tagIndex object where every bracket has it’s index which will help us to identify if we have the open or closing bracket.
– If it is open bracket, we just putting it inside the stack.
– if this is a close bracket, we are using the newly created  tagIndex to figure out if this is the right closing bracket from the same type. Simply following the rule that we described above (lines 32 -35)  `if( tagIndex[symbol] != lastTagIndex + 1 )` Basically we check if the previous opening tag in the stack is of the same type of the current closing tag.

Listen for DOM changes using Mutation Observer

If we ever needed to listen for DOM events, one of the first options that comes to our mind might be to use setTimeout and periodically check the contents of the tag that we are interested in. Like in this example:

function myTest() {
    setTimeout(function() {
        console.log("Beep ...");
        if(document.getElementById('root').innerHTML == 'Loading ...') {
            myTest();
        }
        else {
            // do something 
            alert("READY!");
        }
    }, 1000);
}

But this is intense JavaScript process, which could be done using more elegant way with the help of Mutation Observer. 

Using Mutation Observer we could subscribe for different types of DOM events and trigger function on change.

example code:

<html>
  <body>    
    <div id="root">loading ...</div>

    <script>
      setTimeout(function() {
        document.getElementById('root').innerHTML = "My content is loaded!";
      }, 3000);
    </script>

    <script>
      var targetNode = document.getElementById('root');
      var config = { attributes: true, childList: true, subtree: true, characterData:true };
      var observer = new MutationObserver(function() {
        alert("#root div changed!");  
        // stop observing
        observer.disconnect();      
      });

      observer.observe(targetNode, config);
    </script>
  </body>
</html>

 what is happening here:
– created a div tag with id=’root’ and added ‘Loading …’ text.
– created MutationObserver (line 14) to listen for DOM changes in ‘root’ folder.
– specified which DOM events will trigger the function in config param (line 13)

– after 3 sec. setTimeout event fires and changes the content of the ‘root’ div, triggering DOM change event which will cause the function set in MutationObserver (line 14) to be executed.
– after showing the alert we could optionally disconnect and unsubscribe for DOM change events for this tag.

Here are some of the most used config properties

Mutation observer init properties:

At a minimum, one of childListattributes, and/or characterData must be true when you call observe(). Otherwise, a TypeError exception will be thrown.

attributeFilter Optional
An array of specific attribute names to be monitored. If this property isn’t included, changes to all attributes cause mutation notifications. No default value.
attributeOldValue Optional
Set to true to record the previous value of any attribute that changes when monitoring the node or nodes for attribute changes; see Monitoring attribute values in MutationObserver for details on watching for attribute changes and value recording. No default value.
attributes Optional
Set to true to watch for changes to the value of attributes on the node or nodes being monitored. The default value is false.
characterData Optional
Set to true to monitor the specified target node or subtree for changes to the character data contained within the node or nodes. No default value.
characterDataOldValue Optional
Set to true to record the previous value of a node’s text whenever the text changes on nodes being monitored. For details and an example, see Monitoring text content changes in MutationObserver. No default value.
childList Optional
Set to true to monitor the target node (and, if subtree is true, its descendants) for the addition or removal of new child nodes. The default is false.
subtree Optional
Set to true to extend monitoring to the entire subtree of nodes rooted at target. All of the other MutationObserverInit properties are then extended to all of the nodes in the subtree instead of applying solely to the target node. The default value is false.

Adding routing

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Set up Express server to route all requests to the default ./index.html 

  • For CLI instance this could be achieved by just passing –history-api-fallback in package.json file.
    Give it a try: fire up the project using the cli settings yarn start-cli and go to http://127.0.0.1:8080/test for example and you will see the same homepage no matter what is the url.

./package.json

"scripts": {
  "start-cli": "webpack-dev-server --hot --history-api-fallback",
  "start-api": "babel-node server-api.js",
  "start-middleware": "babel-node server-middleware.js",
  "clean": "rm -rf ./dist",
  "lint": "eslint .",
  "build-dev": "webpack --mode development",
  "build-prod": "webpack --mode production"
},
  • For server using API config we are adding historyApiFallback: true (line 12) which basically tells the the dev server to fallback to the default file if it can’t find a route.

./server-api.js

/**
 * Runs a webpack-dev-server, using the API.
 */
import WebpackDevServer from 'webpack-dev-server';
import webpack from 'webpack';
import config from './webpack.api.config.js';

const compiler = webpack(config);
const server = new WebpackDevServer(compiler, {
  hot: true,
  publicPath: config.output.publicPath,
  historyApiFallback: true
});
server.listen(8080, 'localhost', function() {});
  • For the dev-middleware it a bit different approach since we are no longer using the built in express server in Webpack-dev-server since we have Express server already in place. So the solution is to tell Express server to accept all requests ‘*’ and to server the same index.html for all requests (line 18)

./server-middleware.js

const express = require('express');
const webpackDevMiddleware = require('webpack-dev-middleware');
const webpackHotMiddleware = require('webpack-hot-middleware');
const webpack = require('webpack');
const webpackConfig = require('./webpack.middleware.config.js');
const app = express();
const path = require('path');
const compiler = webpack(webpackConfig);
app.use(webpackDevMiddleware(compiler, {
  hot: true,
  publicPath: webpackConfig.output.publicPath,
}));
app.use(webpackHotMiddleware(compiler, {
  log: console.log,
  path: '/__webpack_hmr',
  heartbeat: 10 * 1000,
}));
app.get('*', function(req, res) {
  res.sendFile(path.join(__dirname, 'index.html'));
});
const server = app.listen(8080, function() {
  const host = server.address().address;
  const port = server.address().port;
  console.log('App is listening at http://%s:%s', host, port);
});

We did this for all server configurations, but at the end we could just stick with only one, but for now we demonstrate how to set up all three configurations.

Let’s continue with setting up React router.

Setting up React router.

Install React-router-dom

yarn add react-router-dom

Before we continue, let’s add another component so we have two URLs to navigate to.

mkdir ./src/Components/About

and create a simple component that will display a message.

./src/Components/About/index.js

import React from 'react';


const About = () => (
  <div>
    <div>Wellcome to my tutorial!</div>
  </div>
)

export default About;

Now let’s move to adding the react router.

There are different palaces where we could ‘wrap’ our project components with Express router component. We could do it in ./src/index.js but I personally prefer to keep ./src/index.js as simple as possible and would move all complex logic into ./src/App/index.js 

./src/App/index.js

import React, { Component } from 'react';
import Greetings from '../Greetings';
import About from '../About';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

import styles from './styles.scss';
export default class App extends Component {
  render() {
    return (
      <div className={styles.appWrapper}>
        <Router>
          <Switch>
            <Route exact path="/greetings" component={() => <Greetings user="John"/> } />
            <Route exact path="/about" component={About} />
          </Switch>
        </Router>
      </div>        
    );
  }
}

what we just did:
– we added React router and Switch module.
– also we added two routes: ‘/greetings’, and ‘/about’ which will open the corresponding components.
– we wrapped ‘Greetings’ component into an inline function that passes the user parameter, otherwise if you try passing it into the route <Route user="John ... React router will simply ignore it.

Wrapping the component into an inline function to pass parameters will work but is not the best approach. The reason for this is because of performance. According to the official docs…

  “When you use the component props, the router uses React.createElement to create a new React element from the given component. That means if you provide an inline function to the component attribute, you would create a new component every render. This results in the existing component unmounting and the new component mounting instead of just updating the existing component.”

So what is the better solution? Instead of using component, use the renderprop. render accepts a functional component and that function won’t get unnecessarily remounted like with component. That function will also receive all the same props that component would receive. So you can take those and pass them along to the rendered component.

Let’s fix this:

./src/App/index.js

import React, { Component } from 'react';
import Greetings from '../Greetings';
import About from '../About';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

import styles from './styles.scss';
export default class App extends Component {
  render() {
    return (
      <div className={styles.appWrapper}>
        <Router>
          <Switch>
            <Route exact path="/greetings" render={(props) => <Greetings {...props} user="John" />} />
            <Route exact path="/about" component={About} />
          </Switch>
        </Router>
      </div>        
    );
  }
}

Give it a try and navigate to http://localhost:8080/about  and http://localhost:8080/greetings

Great! Now we have React router set up with  two pages opening two different components.

Adding Navigation

Let’s create a component that will serve as a navigation menu mkdir ./src/components/Header

./src/components/Header/index.js

import React from 'react';
import { Link } from 'react-router-dom';

const styles = require('./styles.scss');

const Header = ( {title} ) => (
  <div>
    <div className={styles.wrapper}>
      <ul>
        <li><Link to='/home'>HOME</Link></li>
        <li><Link to='/greetings'>GREETINGS</Link></li>       
        <li><Link to='/about'>ABOUT</Link></li>
      </ul>
    </div>
  </div>
);

export default Header;

and let’s add styles:

./src/components/Header/styles.scss

.wrapper {
  
  ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    overflow: hidden;
    border: 1px solid #e7e7e7;
    background-color: #f3f3f3;
  }
  li {
    float: left;
  }

  li a {
    display: block;
    color: #666;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
  }

  li a:hover:not(.active) {
      background-color: #ddd;
  }

  li a.active {
      color: white;
    
  }
}

 

We used React Link component to allow users to navigate through the site.

Now put the component inside the <Router> component.

./src/components/App/index.js

import React, { Component } from 'react';
import Home from '../Home';
import Greetings from '../Greetings';
import About from '../About';
import Header from '../Header';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

import styles from './styles.scss';
export default class App extends Component {
  render() {
    return (
      <div className={styles.appWrapper}>
        <Router>
          <Header />
          <Switch>
            <Route exact path="/home" component={Home} />            
            <Route exact path="/greetings" render={(props) => <Greetings {...props} user="John" />} />
            <Route exact path="/about" component={About} />
          </Switch>
        </Router>
      </div>        
    );
  }
}

You probably noticed that we have 2 pages but the navigation has 3 links. Let’s add another component just to have more pages to navigate through, besides we will need this component later in this tutorial  mkdir ./src/components/Home

./src/components/Home/index.js

import React from 'react';

const styles = require('./styles.scss');

const Home = () => (
  <div>
    <div className={styles.wrapper}>This is my home section!</div>
  </div>
)

export default Home;

./src/components/Home/styles.scss

.wrapper {
  background: rgb(141, 141, 172);
  color: white;
  text-align: center;
  font-family: MyFont;
}

Start the server and give it a try!

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