## Assignment Operators

An assignment operator assigns a value to its left operand based on the value of its right operand.

## Overview

The basic assignment operator is equal (`=`), which assigns the value of its right operand to its left operand. That is, `x = y` assigns the value of `y` to `x`. The other assignment operators are usually shorthand for standard operations, as shown in the following definitions and examples.

## Assignment

Simple assignment operator which assigns a value to a variable. Chaining the assignment operator is possible in order to assign a single value to multiple variables. See the example.

### Syntax

```Operator: x = y
```

### Examples

```// Assuming the following variables
//  x = 5
//  y = 10
//  z = 25

x = y     // x is 10
x = y = z // x, y and z are all 25
```

The addition assignment operator adds the value of the right operand to a variable and assigns the result to the variable. The types of the two operands determine the behavior of the addition assignment operator. Addition or concatenation is possible. See the addition operator for more details.

### Syntax

```Operator: x += y
Meaning:  x  = x + y
```

### Examples

```// Assuming the following variables
//  foo = "foo"
//  bar = 5
//  baz = true

// Number + Number -> addition
bar += 2 // 7

// Boolean + Number -> addition
baz += 1 // 2

// Boolean + Boolean -> addition
baz += false // 1

// Number + String -> concatenation
bar += "foo" // "5foo"

// String + Boolean -> concatenation
foo += false // "foofalse"

// String + String -> concatenation
foo += "bar" // "foobar"
```

## Subtraction assignment

The subtraction assignment operator subtracts the value of the right operand from a variable and assigns the result to the variable. See the subtraction operator for more details.

#### Syntax

```Operator: x -= y
Meaning:  x  = x - y
```

#### Examples

```// Assuming the following variable
//  bar = 5

bar -= 2     // 3
bar -= "foo" // NaN
```

## Multiplication assignment

The multiplication assignment operator multiplies a variable by the value of the right operand and assigns the result to the variable. See the multiplication operator for more details.

### Syntax

```Operator: x *= y
Meaning:  x  = x * y
```

### Examples

```// Assuming the following variable
//  bar = 5

bar *= 2     // 10
bar *= "foo" // NaN
```

## Division assignment

The division assignment operator divides a variable by the value of the right operand and assigns the result to the variable. See the division operator for more details.

### Syntax

```Operator: x /= y
Meaning:  x  = x / y
```

### Examples

```// Assuming the following variable
//  bar = 5

bar /= 2     // 2.5
bar /= "foo" // NaN
bar /= 0     // Infinity
```

## Remainder assignment

The remainder assignment operator divides a variable by the value of the right operand and assigns the remainder to the variable. See the remainder operator for more details.

### Syntax

```Operator: x %= y
Meaning:  x  = x % y
```

### Examples

```// Assuming the following variable
//  bar = 5

bar %= 2     // 1
bar %= "foo" // NaN
bar %= 0     // NaN
```

## Exponentiation assignment

[ECMAScript 2016 (ES7) proposal]

The exponentiation assignment operator returns the result of raising first operand to the power second operand. See the exponentiation operator for more details.

### Syntax

```Operator: x **= y
Meaning:  x  = x ** y
```

### Examples

```// Assuming the following variable
//  bar = 5

bar **= 2     // 25
bar **= "foo" // NaN```

## Left shift assignment

The left shift assignment operator moves the specified amount of bits to the left and assigns the result to the variable. See the left shift operator for more details.

### Syntax

```Operator: x <<= y
Meaning:  x   = x << y
```

### Examples

```var bar = 5; //  (00000000000000000000000000000101)
bar <<= 2; // 20 (00000000000000000000000000010100)
```

## Right shift assignment

The right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the right shift operator for more details.

### Syntax

```Operator: x >>= y
Meaning:  x   = x >> y
```

### Examples

```var bar = 5; //   (00000000000000000000000000000101)
bar >>= 2;   // 1 (00000000000000000000000000000001)

var bar -5; //    (-00000000000000000000000000000101)
bar >>= 2;  // -2 (-00000000000000000000000000000010)
```

## Unsigned right shift assignment

The unsigned right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the unsigned right shift operator for more details.

### Syntax

```Operator: x >>>= y
Meaning:  x    = x >>> y
```

### Examples

```var bar = 5; //   (00000000000000000000000000000101)
bar >>>= 2;  // 1 (00000000000000000000000000000001)

var bar = -5; // (-00000000000000000000000000000101)
bar >>>= 2; // 1073741822 (00111111111111111111111111111110)```

## Bitwise AND assignment

The bitwise AND assignment operator uses the binary representation of both operands, does a bitwise AND operation on them and assigns the result to the variable. See the bitwise AND operator for more details.

### Syntax

```Operator: x &= y
Meaning:  x  = x & y
```

### Example

```var bar = 5;
// 5:     00000000000000000000000000000101
// 2:     00000000000000000000000000000010
bar &= 2; // 0
```

## Bitwise XOR assignment

The bitwise XOR assignment operator uses the binary representation of both operands, does a bitwise XOR operation on them and assigns the result to the variable. See the bitwise XOR operator for more details.

### Syntax

```Operator: x ^= y
Meaning:  x  = x ^ y
```

### Example

```var bar = 5;
bar ^= 2; // 7
// 5: 00000000000000000000000000000101
// 2: 00000000000000000000000000000010
// -----------------------------------
// 7: 00000000000000000000000000000111
```

## Bitwise OR assignment

The bitwise OR assignment operator uses the binary representation of both operands, does a bitwise OR operation on them and assigns the result to the variable. See the bitwise OR operator for more details.

### Syntax

```Operator: x |= y
Meaning:  x  = x | y
```

### Example

```var bar = 5;
bar |= 2; // 7
// 5: 00000000000000000000000000000101
// 2: 00000000000000000000000000000010
// -----------------------------------
// 7: 00000000000000000000000000000111
```

## Examples

### Left operand with another assignment operator

In unusual situations, the assignment operator (e.g.` x += y`) is not identical to the meaning expression (here `x = x + y`). When the left operand of an assignment operator itself contains an assignment operator, the left operand is evaluated only once. For example:

```a[i++] += 5         // i is evaluated only once
a[i++] = a[i++] + 5 // i is evaluated twice
```

Created by Mozilla Contributors and licensed under CC-BY-SA 2.5