Complete JavaScript in 5 mins for Playwright

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Hello World Program in JavaScript

javascript

*// Create a file in Visual Studio Code with .js extension* console.log("Hello World");

To run:

• Open terminal at file location
• Run: node filename.js (make sure Node.js is installed and set in system variables)

Output:

Hello World

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Comments in JavaScript

javascript

// This is a single line comment

/*This is a multi-line comment */

Comments don't impact execution.

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Variables in JavaScript

JavaScript variables are loosely typed - no explicit data types are needed.

Example of declaring a variable:

let a = 4; 

console.log(a); 

// Prints: 4

To check type of variable:

console.log(typeof a); // Outputs: "number"

Variables can be declared with var, let, and const. let and const were introduced in ES6.

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var, let, const

• var: Function scoped, can be redeclared and reassigned.
• let: Block scoped, cannot be redeclared, but can be reassigned.
• const: Block scoped, cannot be redeclared or reassigned (constant).

Example:

let a = 4; a = 5; // OK

let a=6; // cannot be redeclared 

const b = 10; b = 15; // Error: Assignment to constant variable.

var c = 7; var c = 8; // Allowed with var` ,can be redeclared and reassigned.

Use let or const instead of var for better scoping.

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Boolean negation operator

let flag = true; 

console.log(!flag); *// Prints: false*

Negation ! works only with boolean values.

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Reassignment and variable declaration

`javascript

let c = a + b; // Declared once c = a + b; // Reassignment allowed without let keyword

let c = a + b; // Error: 'c' already declared`

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Loops in JavaScript

while loop

let i = 0; 

while (i < 10) 

{ 

 console.log(i);

 i++;

 }

Infinite loop caution: Condition must eventually become false.

• The loop starts with i = 0.
• Each iteration prints i and increases it by 1.
• When i reaches 10, the condition (i < 10) becomes false, so the loop stops.

If you forget to update i, the condition never becomes false, creating an infinite loop.

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do-while loop

javascript

let i = 10; 

do 

{ 

 console.log(i);

 i++; 

} while (i < 10);

Executes once before checking condition.

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for loop

for (let i = 0; i <= 10; i++) 

{ 

 console.log(i); 

}

Use when the number of iterations is known.

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Example: Find multiples of 2 and 5

for (let k = 1; k <= 10; k++) 

{

 if (k % 2 === 0 && k % 5 === 0) 

{

 console.log(k); 

 } 

} *// Output: 10*

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break statement example

let n = 0; 

for (let k = 1; k <= 100; k++) 

{ 

 if (k % 2 === 0 && k % 5 === 0)

 {

 n++;

 console.log(k); 

 if (n === 3) 

break; 

// Stop after first 3 matches* 

 }

 }

• The loop runs from k = 1 to k = 100.
• Inside, the condition (k % 2 === 0 && k % 5 === 0) checks whether k is divisible by both 2 and 5 (which means divisible by 10).
• Each valid match increases n by 1 and prints k.
• Once n reaches 3, the break statement exits the loop early.

So the output stops after printing the first 3 such numbers (10, 20, 30).

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Arrays in JavaScript

Declaring and initializing an array

javascript

let marks = new Array(6);

marks = [20, 40, 35, 45, 50, 37];

Or simply:

javascript

let marks = [20, 40, 35, 45, 50, 37];

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Accessing array elements

javascript

console.log(marks[2]); // Outputs: 35*

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Updating array element

javascript

marks[3] = 14; // Change fourth element to 14

console.log(marks);

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Array length

javascript

console.log(marks.length); // Outputs: 6

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Adding/removing elements

javascript

marks.push(65); // Add 65 at the end* marks.pop(); // Removes last element  marks.unshift(12); // Add 12 at beginning*

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Array methods: indexOf, includes, slice

javascript

console.log(marks.indexOf(35)); // Get index of 35* console.log(marks.includes(120)); // false - if 120 not found

let subMarks = marks.slice(2, 5); // elements from index 2 to 4

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Loop to sum array elements

javascript

let sum = 0;

for(let i = 0; i < marks.length; i++) 

{

 sum += marks[i]; 

}

console.log(sum);

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Using reduce

javascript

let totalMarks = marks.reduce((sum, mark) => sum + mark, 0); 

console.log(totalMarks);

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Filtering even numbers from an array

javascript

let scores = [12, 13, 14, 16]; 

let evenScores = []; 

for(let i = 0; i < scores.length; i++) 

{

 if(scores[i] % 2 === 0) 

{ 

 evenScores.push(scores[i]); 

 } 

} 

console.log(evenScores); *// [12, 14, 16]*

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Using filter method

javascript

let evenScores = scores.filter(score => score % 2 === 0); 

console.log(evenScores);

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Mapping over array elements

Multiply evens by 3:

javascript

let mappedScores = evenScores.map(score => score * 3); 

console.log(mappedScores);

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Combining filter, map, reduce

`javascript

let result = scores .filter(score => score % 2 === 0) .map(score => score * 3) .reduce((total, score) => total + score, 0);

console.log(result);

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Sorting

Sorting strings

javascript

let fruits = ['Banana', 'Mango', 'Apple'];

fruits.sort(); 

console.log(fruits);

Sorting numbers (with custom comparator)

`javascript

let numbers = [12, 3, 19, 16, 14]; 

numbers.sort((a, b) => a - b); 

console.log(numbers); // Ascending order

numbers.sort((a, b) => b - a); 

console.log(numbers); // Descending order`

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Functions in JavaScript

Named function

function add(a, b) 

{ 

 return a + b; 

} 

let result = add(2, 3); 

console.log(result); // 5

Anonymous function assigned to variable

let add = function(a, b) 

{ 

 return a + b; 

}; 

console.log(add(2, 3)); // 5

Arrow function (ES6)

let add = (a, b) => a + b; 

console.log(add(2, 3)); // 5

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Variable scope differences: var vs let vs const

• var is function scoped
• let and const are block scoped

Example problem with var:

`javascript

if (true) {

var grade = "Afternoon"; // var scoped outside block

}

console.log(grade); // Prints "Afternoon"

if (true)

{ 

let grade = "Evening"; 

// let scoped inside block 

} 

console.log(grade); // Error if grade not declared outside block`

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Strings in JavaScript

javascript

let day = "Tuesday"; 

console.log(day.length); // String length

// Substring with slice

 let sub = day.slice(0, 4); // 'Tues'// Access character by index

 console.log(day[1]); // 'u'// Split string

 let parts = day.split('s'); // splits at 's'// Trim whitespace

 let rawString = " hello "; 

console.log(rawString.trim()); // 'hello'

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Converting string to number and vice versa

javascript

let str = "27"; 

let num = parseInt(str);

console.log(num - 23); // Subtract numbers correctly

console.log(num.toString()); // Convert back to string*

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Objects in JavaScript

`javascript

let person = { firstName: "John", lastName: "Doe", gender: "Male" };

// Access properties 

console.log(person.firstName); 

console.log(person['lastName']);

// Update property 

person.firstName = "Tim";

// Add new property 

person.age = 30;

// Delete property 

delete person.gender;

// Check if property exists 

console.log('gender' in person); // false after deletion

// Iterate properties 

for (let key in person)

 { 

console.log(key + ": " + person[key]);

 }

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Object methods (functions as properties)

javascript

let person = {

  firstName: "Tim",

  lastName: "Smith",

  fullName: function() {

    return this.firstName + " " + this.lastName;

  }

};

console.log(person.fullName()); // Call function property

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Classes in JavaScript (ES6)

javascript

class Person {

  constructor(firstName, lastName) {

    this.firstName = firstName; // instance variable

    this.lastName = lastName;   // instance variable

  }

  fullName() {

    return this.firstName + " " + this.lastName;

  }

  get location() {

    return "Canada"; // getter property

  }

}

// Create instance

const p1 = new Person("Tim", "Joseph");

console.log(p1.fullName());

console.log(p1.location);

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Exporting and importing classes in Node.js

Export class:

javascript

module.exports = Person;

Import and use in another file:

javascript

const Person = require('./Person');

const p1 = new Person("Chris", "Edward");

console.log(p1.fullName());

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Inheritance in JavaScript classes

const Person = require('./Person');

class Pet extends Person {

  constructor(firstName, lastName) {

    super(firstName, lastName); // Call parent's constructor

  }

  get location() {

    return "Blue Cross"; // Override parent's location

  }

}

const pet1 = new Pet("Sam", "Sam");

console.log(pet1.fullName());     // Inherited method

console.log(pet1.location);       // Overridden property

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