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Math.trunc()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The Math.trunc() static method returns the integer part of a number by removing any fractional digits.
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
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).
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
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.