Math.round()

The Math.round() function returns the value of a number rounded to the nearest integer.

Syntax

Math.round(x)

Parameters

x
A number.

Return value

The value of the given number rounded to the nearest integer.

Description

If the fractional portion of the argument is greater than 0.5, the argument is rounded to the integer with the next higher absolute value. If it is less than 0.5, the argument is rounded to the integer with the lower absolute value.  If the fractional portion is exactly 0.5, the argument is rounded to the next integer in the direction of +∞.  Note that this differs from many languages' round() functions, which often round this case to the next integer away from zero, instead giving a different result in the case of negative numbers with a fractional part of exactly 0.5.

Because round() is a static method of Math, you always use it as Math.round(), rather than as a method of a Math object you created (Math has no constructor).

Examples

Math.round( 20.49); //  20
Math.round( 20.5 ); //  21
Math.round( 42   ); //  42
Math.round(-20.5 ); // -20
Math.round(-20.51); // -21

Demonstrative Implementation

Below is a snippet of code that is functionally equivelent to math.round except that the snippet of code below is slower than Math.round. The purpose of the snippet of code below is to demonstrate how Math.round works.

function vanilla_round(x) {
    var y = Math.abs(x) + 0.5; // so that less than 1/2 rounds down; greater rounds up
    return Math.floor(x+0.5)
}

The modulus operator above gets the decimal part of x. Further, the above code snippet could be modified to round to a certain precision on a number:

function round_to_precision(x, precision) {
    var y = +x + (precision === undefined ? 0.5 : precision/2);
    return y - (y % (precision === undefined ? 1 : +precision));
}

Examples:

round_to_precision(11, 2); // outputs 12
round_to_precision(11, 3); // outputs 12
round_to_precision(11, 4); // outputs 12
round_to_precision(11, 5); // outputs 10
round_to_precision(11, 6); // outputs 12
round_to_precision(11, 7); // outputs 14
round_to_precision(11, 8); // outputs 8
round_to_precision(3.7, 0.5); // outputs 3.5
round_to_precision(3.75, 0.5); // outputs 4
round_to_precision(3.8, 0.5); // outputs 4

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.0.
ECMAScript 5.1 (ECMA-262)
The definition of 'Math.round' in that specification.
Standard
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Math.round' in that specification.
Standard
ECMAScript Latest Draft (ECMA-262)
The definition of 'Math.round' in that specification.
Draft

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
roundChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 3Opera Full support YesSafari Full support YesWebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support 1.0nodejs Full support Yes

Legend

Full support  
Full support

See also