JavaScript
Statements
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
Example
document.getElementById("demo").innerHTML
= "Hello Dolly.";
»
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:
Example
var x, y, z;
x = 5;
y = 6;
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:
var a, b, c;
a = 5;
b = 6;
c = a + b;
»
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?";
}
»
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.
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