JavaScript Statements

In HTML, JavaScript statements are "instructions" to be "executed" by the web browser.

JavaScript Statements

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


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

Most JavaScript programs contain many JavaScript statements.

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

In this example x, y, and z are given values, and finally z is displayed:


var x = 5;
var y = 6;
var z = x + y;
document.getElementById("demo").innerHTML = z;
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Note 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:

a = 5;
b = 6;
c = a + b;
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When separated by semicolons, multiple statements on one line are allowed:

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


document.getElementById("demo").innerHTML =
"Hello Dolly.";
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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, are in JavaScript functions:


function myFunction() {
    document.getElementById("demo").innerHTML = "Hello Dolly.";
    document.getElementById("myDIV").innerHTML = "How are you?";
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Note In this tutorial we use 4 spaces of indentation for code blocks.
You will learn more about functions later in this tutorial.

JavaScript Keywords

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

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

Note JavaScript keywords are reserved words. Reserved words cannot be used as names for variables.