[05] JavaScript Statements

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

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JavaScript Statements

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

 

<p id="demo"></p> <script>     document.getElementById("demo").innerHTML = "Hello Dolly."; </script>

 

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 is given values, and finally z is displayed:

Example

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

 

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:

 

<p id="demo1"></p> <script>    a = 1;    b = 2;    c = a + b;    document.getElementById("demo1").innerHTML = c; </script>

 

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;

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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, are in JavaScript functions:

Example

function myFunction() {
  document.getElementById("demo").innerHTML = "Hello Dolly.";
  document.getElementById("myDIV").innerHTML = "How are you?";
}

 

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

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