# Math.log2()

The Math.log2() function returns the base 2 logarithm of a number, that is

$∀x>0,Math.log2(x)=log2(x)=the uniqueysuch that2y=x\forall x > 0, \mathtt{\operatorname{Math.log2}(x)} = \log_2(x) = \text{the unique} \; y \; \text{such that} \; 2^y = x$

## Syntax

JavaScript
Math.log2(<var>x</var>)

x
A number.

### Return value

The base 2 logarithm of the given number. If the number is negative, NaN is returned.

## Description

If the value of x is less than 0, the return value is always NaN.

Because log2() is a static method of Math, you always use it as Math.log2(), rather than as a method of a Math object you created (Math is not a constructor).

This function is the equvalent of Math.log(x) / Math.log(2).  For log2(e) use the constant Math.LOG2E which is 1 / Math.LN2.

## Examples

### Using Math.log2()

JavaScript
Math.log2(3);    // 1.584962500721156
Math.log2(2);    // 1
Math.log2(1);    // 0
Math.log2(0);    // -Infinity
Math.log2(-2);   // NaN
Math.log2(1024); // 10

## Polyfill

This Polyfill emulates the Math.log2 function. Note that it returns imprecise values on some inputs (like 1 << 29), wrap into Math.round() if working with bit masks.

JavaScript
Math.log2 = Math.log2 || function(x) {
return Math.log(x) * Math.LOG2E;
};

## Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Math.log2' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Math.log2' in that specification.
Draft

## Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 38 25 (25) No support 25 7.1
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 25.0 (25) No support No support 8