Math.trunc()

Try it

console.log(Math.trunc(13.37));
// Expected output: 13

console.log(Math.trunc(42.84));
// Expected output: 42

console.log(Math.trunc(0.123));
// Expected output: 0

console.log(Math.trunc(-0.123));
// Expected output: -0

Syntax

Math.trunc(x)

Parameters

x

A number.

Return value

The integer part of x.

Description

The way Math.trunc() works is more straightforward than the other three Math methods: Math.floor(), Math.ceil() and Math.round(); 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

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

See also

• Polyfill of Math.trunc in core-js
• Math.abs()
• Math.ceil()
• Math.floor()
• Math.round()
• Math.sign()