JavaScript Statements
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JavaScript Statements
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Example
var x, y, z; // Statement 1
x = 5; // Statement 2
y = 6; // Statement 3
z = x + y; // Statement 4
Try it Yourself »
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.";Try it Yourself »
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
Try it Yourself »
When separated by semicolons, multiple statements on one line are allowed:
a = 5; b = 6; c = a + b;
Try it Yourself »
On the web, you might see examples without semicolons.
Ending statements with semicolon is not required, but highly recommended.
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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!";Try it Yourself »
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?";
Try it Yourself »
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.
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