Math.trunc() - JavaScript | MDN
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
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()
js
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:
js
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.
js
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
| Specification |
|---|
| ECMAScript® 2027 Language Specification # sec-math.trunc |