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The Math.round()
function returns the value of a number rounded to the nearest integer.
Syntax
Math.round(x)
Parameters
x
- A number.
Return value
The value of the given number rounded to the nearest integer.
Description
If the fractional portion of the argument is 0.5 or greater, the argument is rounded to the integer with the next higher absolute value. Otherwise, the argument is rounded to the integer with the lower absolute value.
Because round()
is a static method of Math
, you always use it as Math.round()
, rather than as a method of a Math
object you created (Math
has no constructor).
Examples
Math.round( 20.49); // 20 Math.round( 20.5); // 21 Math.round( 42 ); // 42 Math.round(-20.5); // -20 Math.round(-20.51); // -21
Decimal rounding
// Closure (function() { /** * Decimal adjustment of a number. * * @param {String} type The type of adjustment. * @param {Number} value The number. * @param {Integer} exp The exponent (the 10 logarithm of the adjustment base). * @returns {Number} The adjusted value. */ function decimalAdjust(type, value, exp) { // If the exp is undefined or zero... if (typeof exp === 'undefined' || +exp === 0) { return Math[type](value); } value = +value; exp = +exp; // If the value is not a number or the exp is not an integer... if (isNaN(value) || !(typeof exp === 'number' && exp % 1 === 0)) { return NaN; } // Shift value = value.toString().split('e'); value = Math[type](+(value[0] + 'e' + (value[1] ? (+value[1] - exp) : -exp))); // Shift back value = value.toString().split('e'); return +(value[0] + 'e' + (value[1] ? (+value[1] + exp) : exp)); } // Decimal round if (!Math.round10) { Math.round10 = function(value, exp) { return decimalAdjust('round', value, exp); }; } // Decimal floor if (!Math.floor10) { Math.floor10 = function(value, exp) { return decimalAdjust('floor', value, exp); }; } // Decimal ceil if (!Math.ceil10) { Math.ceil10 = function(value, exp) { return decimalAdjust('ceil', value, exp); }; } })(); // Round Math.round10(55.55, -1); // 55.6 Math.round10(55.549, -1); // 55.5 Math.round10(55, 1); // 60 Math.round10(54.9, 1); // 50 Math.round10(-55.55, -1); // -55.5 Math.round10(-55.551, -1); // -55.6 Math.round10(-55, 1); // -50 Math.round10(-55.1, 1); // -60 Math.round10(1.005, -2); // 1.01 -- compare this with Math.round(1.005*100)/100 above // Floor Math.floor10(55.59, -1); // 55.5 Math.floor10(59, 1); // 50 Math.floor10(-55.51, -1); // -55.6 Math.floor10(-51, 1); // -60 // Ceil Math.ceil10(55.51, -1); // 55.6 Math.ceil10(51, 1); // 60 Math.ceil10(-55.59, -1); // -55.5 Math.ceil10(-59, 1); // -50
PHP-Like rounding Method
The code below can be used to add your own version of Math.round to your own namespace which takes a precision parameter. Unlike Decimal rounding in the example above, this performs no conversion to and from strings, and the precision parameter works same way as PHP and Excel whereby a positive 1 would round to 1 decimal place and -1 would round to the tens.
var myNamespace = {}; myNamespace.round = function(number, precision) { var factor = Math.pow(10, precision); var tempNumber = number * factor; var roundedTempNumber = Math.round(tempNumber); return roundedTempNumber / factor; }; myNamespace.round(1234.5678, 1); // 1234.6 myNamespace.round(1234.5678, -1); // 1230
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. Implemented in JavaScript 1.0. |
ECMAScript 5.1 (ECMA-262) The definition of 'Math.round' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Math.round' in that specification. |
Standard | |
ECMAScript 2017 Draft (ECMA-262) The definition of 'Math.round' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Firefox Mobile (Gecko) | IE Phone | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |