if...else - JavaScript 编辑

The if statement executes a statement if a specified condition is truthy. If the condition is falsy, another statement can be executed.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

if (condition)
   statement1
[else
   statement2]

condition

An expression that is considered to be either truthy or falsy.

statement1

Statement that is executed if condition is truthy. Can be any statement, including further nested if 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 is falsy and the else clause exists. Can be any statement, including block statements and further nested if 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 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 truthiness or falsiness of the Boolean object. Any value that is not false, undefined, null, 0, -0, NaN, or the empty string (""), and any object, including a Boolean object whose value is false, is considered truthy when used as the condition. For example:

var b = new Boolean(false);
if (b) // this condition is truthy

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 > 50) {
  /* do something */
} else if (x > 5) {
  /* do something */
} else {
  /* do something */
}

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 something */
}

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 something */
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'if statement' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• block
• switch
• Conditional operator