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Math.trunc()

The Math.trunc() static method returns the integer part of a number by removing any fractional digits.

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Syntax

Math.trunc(x)

Parameters

x

A number.

Return value

The integer part of x.

Description

Unlike the other three Math methods: Math.floor(), Math.ceil() and Math.round(), the way Math.trunc() works is very simple. It truncates (cuts off) the dot and the digits to the right of it, no matter whether the argument is a positive or negative number.

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

Examples

Using Math.trunc()

Math.trunc(-Infinity); // -Infinity
Math.trunc("-1.123"); // -1
Math.trunc(-0.123); // -0
Math.trunc(-0); // -0
Math.trunc(0); // 0
Math.trunc(0.123); // 0
Math.trunc(13.37); // 13
Math.trunc(42.84); // 42
Math.trunc(Infinity); // Infinity

Using bitwise no-ops to truncate numbers

Warning: This is not a polyfill for Math.trunc() because of non-negligible edge cases.

Bitwise operations convert their operands to 32-bit integers, which people have historically taken advantage of to truncate float-point numbers. Common techniques include:

const original = 3.14;
const truncated1 = ~~original; // Double negation
const truncated2 = original & -1; // Bitwise AND with -1
const truncated3 = original | 0; // Bitwise OR with 0
const truncated4 = original ^ 0; // Bitwise XOR with 0
const truncated5 = original >> 0; // Bitwise shifting by 0

Beware that this is essentially toInt32, which is not the same as Math.trunc. When the value does not satisfy -231 - 1 < value < 231 (-2147483649 < value < 2147483648), the conversion would overflow.

const a = ~~2147483648; // -2147483648
const b = ~~-2147483649; // 2147483647
const c = ~~4294967296; // 0

Only use ~~ as a substitution for Math.trunc() when you are confident that the range of input falls within the range of 32-bit integers.

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
trunc 38 12 25 No 25 8 38 38 25 25 8 3.0 1.0 0.12.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/trunc