Lets start with a quick revision of map() and filter() and some exercises 🔥

• map() iterates over each element of the array and returns a new array with the results of the callback function
• filter() returns true false for each element of the array and returns a new array with the elements that return true

Question 01

• Get the number of products whose price is atleast 100.

const products = [
  { name: "T-Shirt", price: 25 },
  { name: "Headphones", price: 125 },
  { name: "Keyboard", price: 75 },
  { name: "Monitor", price: 200 },
];
// hint : chaining map and filter

Answer:

let productsAbove100 = products
  .map(function (product) {
    return product.price;
  })
  .filter(function (price) {
    return price >= 100;
  });
console.log(productsAbove100.length); // 2

Question 02

• Get the movie Names from this Array of Objects method and only get the movie name which has rating higher than or equal to 8
• Hint : Use Filter and map chaining to achaive the objective

var newReleases = [
  {
    id: 1,
    title: "Die Hard",
    rating: 9,
  },
  {
    id: 2,
    title: "Bad Boys",
    rating: 7,
  },
  {
    id: 3,
    title: "The Chamber",
    rating: 10,
  },
];

Solution using filter and map chaining

let highRatedMovies = newReleases
  .filter(function (newRelease) {
    return newRelease.rating >= 8;
  })
  .map(function (movie) {
    return movie.title;
  });
console.log(highRatedMovies);

Output:

[ 'Die Hard', 'The Chamber' ]

Question 03

• Map Question
  • 1. You have to use map function and have to get all the students name in upperCase
• Use filter method to approach the problem
  • 1. Retrieve the details of students who scored more than 50 marks and have id greater than 120 from studentRecord

let studentRecords = [
  { name: "Abhishek", id: 123, marks: 98 },
  { name: "Udai", id: 101, marks: 90 },
  { name: "Himanshu", id: 200, marks: 96 },
  { name: "Mrinal", id: 115, marks: 75 },
];

• Solution 1

let upperStudents = studentRecords.map(function (student) {
  return student.name.toUpperCase();
});
console.log(upperStudents);

Output :

[ 'ABHISHEK', 'UDAI', 'HIMANSHU', 'MRINAL' ]

• Solution 2

let highScoreStudents = studentRecords.filter(function (student) {
  return student.marks > 50 && student.id > 120;
});
console.log(highScoreStudents);

Output :

[
  { name: 'Abhishek', id: 123, marks: 98 },
  { name: 'Himanshu', id: 200, marks: 96 }
]

Next Higer Order Function | reduce()

• reduce() reduces the array to a single value

  The reduce() method executes a user-supplied “reducer” callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value. The first time that the callback is run there is no "return value of the previous calculation". If supplied, an initial value may be used in its place. Otherwise the array element at index 0 is used as the initial value and iteration starts from the next element (index 1 instead of index 0).

• Perhaps the easiest-to-understand case for reduce() is to return the sum of all the elements in an array:

const array1 = [1, 2, 3, 4];

// 0 + 1 + 2 + 3 + 4
const initialValue = 0;
const sumWithInitial = array1.reduce(
  (previousValue, currentValue) => previousValue + currentValue,
  initialValue
);

console.log(sumWithInitial);
// expected output: 10

The reducer walks through the array element-by-element, at each step adding the current array value to the result from the previous step (this result is the running sum of all the previous steps) — until there are no more elements to add.

Syntax

// Arrow function
reduce((previousValue, currentValue) => {
  /* ... */
});
reduce((previousValue, currentValue, currentIndex) => {
  /* ... */
});
reduce((previousValue, currentValue, currentIndex, array) => {
  /* ... */
});
reduce((previousValue, currentValue, currentIndex, array) => {
  /* ... */
}, initialValue);

// Callback function
reduce(callbackFn);
reduce(callbackFn, initialValue);

// Inline callback function
reduce(function (previousValue, currentValue) {
  /* ... */
});
reduce(function (previousValue, currentValue, currentIndex) {
  /* ... */
});
reduce(function (previousValue, currentValue, currentIndex, array) {
  /* ... */
});
reduce(function (previousValue, currentValue, currentIndex, array) {
  /* ... */
}, initialValue);

Example

const array1 = [1, 2, 3, 4];

let addition = array1.reduce(function (sum, value) {
  let updatedSum = sum + value;
  return updatedSum;
});
console.log(addition); // expected output: 10

let multiplication = array1.reduce(function (product, value) {
  let updatedProduct = product * value;
  return updatedProduct;
});
console.log(multiplication); // expected output: 24

Output :

> node reduce.js
10
24

Question 01 - Reduce

• From the Transactions Array filter out positive Elements and Calculate the total amount
• Use filter and Reduce to Achieve this

const transactions = [1000, 3000, 4000, 2000, -898, 3800, -4500];

let totalPositiveTransactions = transactions
  .filter(function (transaction) {
    return transaction > 0;
  })
  .reduce(function (sum, value) {
    return sum + value;
  });
console.log(totalPositiveTransactions); // expected output: 13800

Output :

> node reduce.js
13800

Reduce Right | reduceRight()

• reduceRight() - it just like reduce() but it goes from right to left

const arr = [1, 2, 3, 4];
let sum = arr.reduceRight(function (sum, value) {
  let updatedSum = sum + value;
  console.log(updatedSum); // 7, 9, 10
  return updatedSum;
});
console.log(sum); // expected output: 10

Output :

> node reduceRight.js
7
9
10
10

forEach() and findIndex()

• forEach() - it is used to iterate over the array and execute a callback function on each element of the array
• findIndex() - it is used to find the index of the first element in the array that satisfies the provided testing function

Object Oriented JavaScript | this keyword

• this keyword is used to access the current object in the context of the function it is used in.

  Js has two modes strict and non-strict

"use strict"; // strict mode
a = 10; // error
console.log(a); // not defined

In non-strict mode, the this keyword refers to the global object. In strict mode, the this keyword refers to the object that is currently executing the function.

• Js has two environments:

  • Browser
  • Node

Context : this in Node Environment with non strict mode

• this in global area returns empty object {}

console.log(this); // {} empty object

Output :

> node this.js
{}

• this in function area returns global object

function f() {
  console.log(this); // function object : {}
}
f();

Output :

> node this.js
<ref *1> Object [global] {
  global: [Circular *1],
  clearInterval: [Function: clearInterval],
  clearTimeout: [Function: clearTimeout],
  setInterval: [Function: setInterval],
  setTimeout: [Function: setTimeout] {
    [Symbol(nodejs.util.promisify.custom)]: [Getter]
  },
  queueMicrotask: [Function: queueMicrotask],
  performance: Performance {
    nodeTiming: PerformanceNodeTiming {
      name: 'node',
      entryType: 'node',
      startTime: 0,
      duration: 231.9817570000887,
      nodeStart: 50.90881999954581,
      v8Start: 82.18204599991441,
      bootstrapComplete: 200.12776999920607,
      environment: 133.09282599948347,
      loopStart: -1,
      loopExit: -1,
      idleTime: 0
    },
    timeOrigin: 1644687300184.629
  },
  clearImmediate: [Function: clearImmediate],
  setImmediate: [Function: setImmediate] {
    [Symbol(nodejs.util.promisify.custom)]: [Getter]
  }
}

• this in object in function area returns object itself

let obj = {
  a: 10,
  f: function () {
    console.log(this); // obj
  },
};
obj.f();

Output :

{ a: 10, f: [Function: f] }

• Function inside Function inside Object | this keyword returns the global object

let obj = {
  a: 10,
  f: function () {
    let f = function () {
      console.log(this); // obj
    };
    f();
  },
};
obj.f();

<ref *1> Object [global] {
  global: [Circular *1],
  clearInterval: [Function: clearInterval],
  clearTimeout: [Function: clearTimeout],
  setInterval: [Function: setInterval],
  setTimeout: [Function: setTimeout] {
    [Symbol(nodejs.util.promisify.custom)]: [Getter]
  },
  queueMicrotask: [Function: queueMicrotask],
  performance: Performance {
    nodeTiming: PerformanceNodeTiming {
      name: 'node',
      entryType: 'node',
      startTime: 0,
      duration: 81.35366600006819,
      nodeStart: 4.461749998852611,
      v8Start: 8.080905999988317,
      bootstrapComplete: 42.207240998744965,
      environment: 26.03384899906814,
      loopStart: -1,
      loopExit: -1,
      idleTime: 0
    },
    timeOrigin: 1644687676750.476
  },
  clearImmediate: [Function: clearImmediate],
  setImmediate: [Function: setImmediate] {
    [Symbol(nodejs.util.promisify.custom)]: [Getter]
  }
}

Node Environment | this keyword in Node Environment with strict mode

// Node + strict mode
"use strict";

// global context
console.log(this); // {} empty object

// function context
function f() {
  console.log(this); // function object : {}
}
f(); // undefined

// object context
let obj = {
  a: 10,
  f: function () {
    console.log(this); // obj
  },
};
obj.f(); // obj itself

// object context 2
let obj2 = {
  a: 20,
  f: function () {
    function g() {
      console.log(this);
    }
    g();
  },
};
obj2.f(); // undefined

Browser Environment | this keyword in Browser Environment with non strict mode

// Browser + Non Strict Mode

// Global Context
console.log(this); // Window object gets returned

// Function Context
function f() {
  console.log(this); // Window object gets returned
}
f();

// Object Context
let obj = {
  a: 2,
  b: function () {
    console.log(this); // Object gets returned
  },
};
obj.b(); // Object itself gets returned

// Object Function Function Context
let obj2 = {
  a: 2,
  b: function () {
    function f() {
      console.log(this); // Window object gets returned
    }
    f();
  },
};
obj2.b(); // Window object gets returned

Browser Environment | this keyword in Browser Environment with strict mode

// Browser + Strict Mode

"use strict";

// Global Context
console.log(this); // Window object gets returned

// Function Context
function f() {
  console.log(this); // undefined gets returned
}
f();

// Object Context
let obj = {
  a: 2,
  b: function () {
    console.log(this); // Object gets returned
  },
};
obj.b(); // Object itself gets returned

// Object Function Function Context
let obj2 = {
  a: 2,
  b: function () {
    function f() {
      console.log(this);
    }
    f();
  },
};
obj2.b(); // undefined gets returned

Table of values of this keyword in different environments and different modes

• Node Environment

• Browser Environment