JavaScript Statements

Example

var x, y, z; // Statement 1

x = 5; // Statement 2

y = 6; // Statement 3

z = x + y; // Statement 4

JavaScript Programs

A computer program is a list of "instructions" to be "executed" by a computer.

In a programming language, these programming instructions are called statements.

A JavaScript program is a list of programming statements.

In HTML, JavaScript programs are executed by the web browser.

JavaScript Statements

JavaScript statements are composed of:

Values, Operators, Expressions, Keywords, and Comments.

This statement tells the browser to write "Hello Dolly." inside an HTML element with id="demo":

Example

document.getElementById("demo").innerHTML = "Hello Dolly.";

Most JavaScript programs contain many JavaScript statements.

The statements are executed, one by one, in the same order as they are written.

JavaScript programs (and JavaScript statements) are often called JavaScript code.

Semicolons ;

Semicolons separate JavaScript statements.

Add a semicolon at the end of each executable statement:

var a, b, c; // Declare 3 variables

a = 5; // Assign the value 5 to a

b = 6; // Assign the value 6 to b

c = a + b; // Assign the sum of a and b to c

When separated by semicolons, multiple statements on one line are allowed:

a = 5; b = 6; c = a + b;

On the web, you might see examples without semicolons.

Ending statements with semicolon is not required, but highly recommended.

JavaScript White Space

JavaScript ignores multiple spaces. You can add white space to your script to make it more readable.

The following lines are equivalent:

var person = "Hege";

var person="Hege";

A good practice is to put spaces around operators ( = + - * / ):

var x = y + z;

JavaScript Line Length and Line Breaks

For best readability, programmers often like to avoid code lines longer than 80 characters.

If a JavaScript statement does not fit on one line, the best place to break it is after an operator:

Example

document.getElementById("demo").innerHTML =

"Hello Dolly!";

JavaScript Code Blocks

JavaScript statements can be grouped together in code blocks, inside curly brackets {...}.

The purpose of code blocks is to define statements to be executed together.

One place you will find statements grouped together in blocks, is in JavaScript functions:

Example

function myFunction() {

document.getElementById("demo1").innerHTML = "Hello Dolly!";

document.getElementById("demo2").innerHTML = "How are you?";

JavaScript Keywords

JavaScript statements often start with a keyword to identify the JavaScript action to be performed.

Visit our Reserved Words reference to view a full list of JavaScript keywords.

Here is a list of some of the keywords you will learn about in this tutorial:

Keyword

Description

break

Terminates a switch or a loop

continue

Jumps out of a loop and starts at the top

debugger

Stops the execution of JavaScript, and calls (if available) the debugging function

do ... while

Executes a block of statements, and repeats the block, while a condition is true

for

Marks a block of statements to be executed, as long as a condition is true

function

Declares a function

if ... else

Marks a block of statements to be executed, depending on a condition

return

Exits a function

switch

Marks a block of statements to be executed, depending on different cases

try ... catch

Implements error handling to a block of statements

var

Declares a variable