if...else
The if statement executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
Syntax
if (condition) statement1 [else statement2]
condition
- An expression that evaluates to true or false.
statement1
- Statement that is executed if
condition
evaluates to true. Can be any statement, including further nestedif
statements. To execute multiple statements, use a block statement ({ ... }) to group those statements, to execute no statements, use an empty statement.
statement2
- Statement that is executed if
condition
evaluates to false and theelse
clause exists. Can be any statement, including block statements and further nestedif
statements.
Description
Multiple if...else
statements can be nested to create an else if
clause. Note that there is no elseif
(in one word) keyword in JavaScript.
if (condition1) statement1 else if (condition2) statement2 else if (condition3) statement3 ... else statementN
To see how this works, this is how it would look like if the nesting were properly indented:
if (condition1) statement1 else if (condition2) statement2 else if (condition3) ...
To execute multiple statements within a clause, use a block statement ({ ... }
) to group those statements. In general, it is a good practice to always use block statements, especially in code involving nested if
statements:
if (condition) { statements1 } else { statements2 }
Do not confuse the primitive boolean values true
and false
with the true and false values of the Boolean object. Any value that is not undefined
, null
, 0
, NaN
, or the empty string (""
), and any object, including a Boolean object whose value is false, evaluates to true when passed to a conditional statement. For example:
var b = new Boolean(false); if (b) // this condition evaluates to true
Examples
Using if...else
if (cipher_char === from_char) { result = result + to_char; x++; } else { result = result + clear_char; }
Using else if
Note that there is no elseif
syntax in JavaScript. However, you can write it with a space between else
and if
:
if (x > 5) { } else if (x > 50) { } else { }
Assignment within the conditional expression
It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:
if (x = y) { /* do the right thing */ }
If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:
if ((x = y)) { /* do the right thing */ }
Specifications
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/statements/if...else