Math.trunc()

The Math.trunc() function 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 the given number.

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.

The argument passed to this method will be converted to number type implicitly.

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).

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 -2^31 - 1 < value < 2^31 (-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.

Examples

Using Math.trunc()

Math.trunc(13.37);    // 13
Math.trunc(42.84);    // 42
Math.trunc(0.123);    //  0
Math.trunc(-0.123);   // -0
Math.trunc('-1.123'); // -1
Math.trunc(NaN);      // NaN
Math.trunc('foo');    // NaN
Math.trunc();         // NaN

Specifications

Specification
ECMAScript Language Specification
# sec-math.trunc

Browser compatibility

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See also