JavaScript Keywords Reference

Keyword Categories

let, const, var
function, class

if, else, switch
for, while, do

true, false, null
undefined, this

new, delete, typeof
instanceof, in

🔹 Declaration Keywords

Keywords used to declare variables, functions, and classes:

🔸 Variable Declaration

// let - declares a block-scoped variable
let userName = "Alice";
let age = 25;

// const - declares a constant
const PI = 3.14159;
const colors = ["red", "green", "blue"];

// var - declares a function-scoped variable (older style)
var oldVariable = "avoid using var";

console.log(userName + " is " + age + " years old");

🔸 Function Declaration

// function - declares a function
function greetUser(name) {
    return "Hello, " + name + "!";
}

// class - declares a class
class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }
}

let greeting = greetUser("Bob");
let person = new Person("Charlie", 30);
console.log(greeting);

🔹 Control Flow Keywords

Keywords that control how your program executes:

🔸 Conditional Keywords

let score = 85;

// if, else - conditional execution
if (score >= 90) {
    console.log("Grade: A");
} else if (score >= 80) {
    console.log("Grade: B");
} else {
    console.log("Grade: C or below");
}

// switch, case, default - multiple conditions
let day = "Monday";
switch (day) {
    case "Monday":
        console.log("Start of work week");
        break;
    case "Friday":
        console.log("TGIF!");
        break;
    default:
        console.log("Regular day");
}

🔸 Loop Keywords

// for - counted loop
for (let i = 1; i <= 3; i++) {
    console.log("Loop iteration: " + i);
}

// while - conditional loop
let count = 0;
while (count < 2) {
    console.log("While loop: " + count);
    count++;
}

// do...while - loop that runs at least once
let num = 0;
do {
    console.log("Do-while: " + num);
    num++;
} while (num < 1);

🔹 Value Keywords

Keywords that represent special values:

// true, false - boolean values
let isActive = true;
let isComplete = false;

// null - intentional absence of value
let data = null;

// undefined - variable declared but not assigned
let notAssigned;

// this - refers to current object context
let person = {
    name: "John",
    greet: function() {
        return "Hi, I'm " + this.name;
    }
};

console.log("Active: " + isActive);
console.log("Data: " + data);
console.log("Not assigned: " + notAssigned);
console.log(person.greet());

🔹 Operation Keywords

Keywords that perform special operations:

🔸 Object Operations

// new - creates new object instance
let date = new Date();
let array = new Array(1, 2, 3);

// delete - removes object property
let car = {brand: "Toyota", model: "Camry", year: 2020};
delete car.year;

// typeof - returns type of variable
let name = "Alice";
let age = 25;
console.log("Name type: " + typeof name);
console.log("Age type: " + typeof age);

🔸 Comparison Operations

// instanceof - checks if object is instance of class
let numbers = [1, 2, 3];
console.log("Is array: " + (numbers instanceof Array));

// in - checks if property exists in object
let person = {name: "Bob", age: 30};
console.log("Has name: " + ("name" in person));
console.log("Has height: " + ("height" in person));

🔹 Function Control Keywords

Keywords that control function execution:

// return - exits function and returns value
function calculateArea(width, height) {
    if (width <= 0 || height <= 0) {
        return 0; // Early return for invalid input
    }
    return width * height;
}

// break - exits loop or switch
for (let i = 0; i < 10; i++) {
    if (i === 3) {
        break; // Exit loop when i equals 3
    }
    console.log("Number: " + i);
}

// continue - skips current iteration
for (let i = 0; i < 5; i++) {
    if (i === 2) {
        continue; // Skip when i equals 2
    }
    console.log("Count: " + i);
}

let area = calculateArea(5, 4);
console.log("Area: " + area);

🔹 Complete Keywords List

All JavaScript keywords organized by category: