JavaScript Basics

By Xah Lee. Date: . Last updated: .

This page teaches you the basics of JavaScript, for beginners, in 30 minutes.

This tutorial teachs the ES2015 version of JavaScript.

ES2015 is a new JavaScript spec released in 2015, and is supported by all browsers now. It adds significant features to the language.

Running JavaScript

How to Run JavaScript

Comment

All lines beginning with 2 slashs // are ignored.

And any text between /* and */ are ignored.

// this is single line comment

/* this is
multi-line
comment. */

Printing

To print, use alert or console.log.

alert("hello");
// bring up a pop-up dialog
console.log("hello");
// print to browser's console

To see the output of console.log, open your browser's JavaScript console.

[see How to Use Browser Console]

Semicolon can be omitted most of the time. When to insert semicolon is complex. For now, just follow examples in this tutorial. For detail, see Semicolon Rules.

String

Quoting String

Use double quote or single quote for string. There's basically no difference between the two.

let s1 = "Mary's cat";  // double quote
let s2 = 'Mary\'s cat'; // single quote, with escape

Use \n for newline.

// use \n for newline
let ss = "cat\ndog";

console.log(ss); // prints 2 lines

Literal newline in string is not allowed. Use backslash to continue a line.

// illegal syntax. Literal newline is not allowed
let s = "a
b";

// use backslash to continue a line
let s = "c\
d";

console.log(s); // prints cd

String length

"abc".length; // 3

Note: no parenthesis after “length”. (because “length” is a “property”, not “method”.)

[see Property Overview]

Substring

string.slice(begin_index, end_index)
Return a substring from begin_index to end_index, not including the end_index.

Index starts at 0.

[see String.prototype.slice]

console.log( "01234".slice(1,3) === "12"); // true

Join String

String join. Use the plus operator +.

console.log( "tiger" + "rabbit"); // tigerrabbit

String Methods

There are many string functions. For a full list, see:

String.prototype

For detail about string, see

String Overview

Arithmetic Operators

Basic arithmetic.

3 + 4; // 7
3 - 4; // -1
3 * 4; // 12
3 / 4; // 0.75

// remainder (mod)
10 % 3; // 1

// negation
-(3+4); // -7

// power
2**3 // 8

// square root
console.log( 
Math.sqrt(4) === 2
);
// true

// cube root
console.log( 
Math.cbrt(8) === 2
);
// true

// 4th root
console.log( 
Math.pow(16, 1/4) === 2
);
// true

[see Math]

Assignment Operators

Assignment lets you store a value to a variable.

let z = 5; // 5
console.log( z ); // 5

Assignment returns a value.

// assignment returns a value
let z;
console.log( z = 2 ); // 2

JavaScript also supports x++ and others.

[see Operators]

Number

JavaScript doesn't have int/float distinction. They are just type “number”. It's similar to other language's “float”.

console.log(typeof 3 === "number");
console.log(typeof 3.2 === "number");
// true

Convert Decimal to Integer

Use one of

to convert a number to integer.

let x = 3.5847;

let f = Math.floor( x );
let c = Math.ceil( x );

console.log(f); // prints 3
console.log(c); // prints 4

console.log(Math.round( 3.54 )); // prints 4
console.log(Math.round( 3.55 )); // prints 4

[see Math]

String/Number Conversion

String to Number

Number to String

Number.prototype.toString

Variables

The following are ways to declare and or assign variable.

// declare variable
let x;
// declare multiple variables
let a, b, c;
// declare variable and assign
let x = 4;
// declare variable and assign multiple variables
let a = 1, b = 2, c = 3;
// declare multiple variables, some with value
let a, b = 2, c;

[see let Declaration]

Besides let, there's also const and var. They all have the same syntax.

const is like let, except that it must be assigned a value, and once assigned, it cannot be changed.

[see const Declaration]

var is like let, but has complex scoping rules and has name hoisting. var is deprecated in JS2015. You should never use it.

[see var Declaration]

Data Types

Every JavaScript value has a type.

JavaScript value types are:

[see Value Types]

True and False

true and false are builtin boolean data types.

Zero, empty string, undefined, null, NaN, eval to false in a “if” statement. Everything else eval to true.

[see true, false]

Logic Operators

// The “and” operator
true && true //  true

// The “or” operator
true || true //  true

// The “negation” operator
!true // false

Comparison Operators

3 < 4;         // true
3 > 4;         //  false

3 == 3;        // true.
3 == "3";      // true.  note: 「==」 does automatic type conversion
3 === "3";     // false.  True if both sides are same type and equal

3 != 4;        // true. The 「!=」 is the negation of 「==」
3 !== 4;       // true. The 「!==」 is the negation of 「===」

What is the difference between == and ===?

== does automatic type conversion. === does not.

You should always use ===.

If Then Else

Simple “if” statement.

if (3 < 4) {console.log("yes");}

“if else” statement.

if (3 <= 4) {console.log("yes");}
else {console.log("no");}

“else if” chain.

let x = 3;
if (x == 1) {console.log("is 1");}
else if (x == 2) {console.log("is 2");}
else if (x == 3) {console.log("is 3");}
else {console.log("not found");}

[see Branch Control: if then else, switch]

Iteration; Loop

The most useful loop syntax is “for” loop.

for (let i=0; i < 9; i++) { console.log(i); }

[see for while do Loop]

Array

Array Basics

Define Function

Example of defining a function:

function ff(x, y) { return x + y;}
console.log(ff(3, 4)); // 7

If the function does not have a return statement, it returns undefined.

JavaScript is a functional language. The definition of a function returns a value of type “function” that represent the function. You can pass a function as argument into a function. Function can return a function. You can also have nested function. Understanding how functions can be passed around is essential, because it is heavily used in JavaScript code.

For detail, see: Functional Programing

JavaScript Object

Object Basics

JS in Depth
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JS Obj Ref

DOM


JS in Depth

Basic Syntax

Value Types

Variable

String

Function

Property

Object and Inheritance

Array

Constructor/Class

Iterable 🌟

Regular Expression

Date

Set Object

Map Object

Number

Misc