Truncate (not round off) decimal numbers in javascript

Question

I am trying to truncate decimal numbers to decimal places. Something like this:

5.467   -> 5.46   985.943 -> 985.94 

toFixed(2) does just about the right thing but it rounds off the value. I don't need the value rounded off. Hope this is possible in javascript.

Answer 1

Dogbert's answer is good, but if your code might have to deal with negative numbers, Math.floor by itself may give unexpected results.

E.g. Math.floor(4.3) = 4, but Math.floor(-4.3) = -5

Use a helper function like this one instead to get consistent results:

truncateDecimals = function (number) {     return Math[number < 0 ? 'ceil' : 'floor'](number); };  // Applied to Dogbert's answer: var a = 5.467; var truncated = truncateDecimals(a * 100) / 100; // = 5.46 

Here's a more convenient version of this function:

truncateDecimals = function (number, digits) {     var multiplier = Math.pow(10, digits),         adjustedNum = number * multiplier,         truncatedNum = Math[adjustedNum < 0 ? 'ceil' : 'floor'](adjustedNum);      return truncatedNum / multiplier; };  // Usage: var a = 5.467; var truncated = truncateDecimals(a, 2); // = 5.46  // Negative digits: var b = 4235.24; var truncated = truncateDecimals(b, -2); // = 4200 

If that isn't desired behaviour, insert a call to Math.abs on the first line:

var multiplier = Math.pow(10, Math.abs(digits)), 

EDIT: shendz correctly points out that using this solution with a = 17.56 will incorrectly produce 17.55. For more about why this happens, read What Every Computer Scientist Should Know About Floating-Point Arithmetic. Unfortunately, writing a solution that eliminates all sources of floating-point error is pretty tricky with javascript. In another language you'd use integers or maybe a Decimal type, but with javascript...

This solution should be 100% accurate, but it will also be slower:

function truncateDecimals (num, digits) {     var numS = num.toString(),         decPos = numS.indexOf('.'),         substrLength = decPos == -1 ? numS.length : 1 + decPos + digits,         trimmedResult = numS.substr(0, substrLength),         finalResult = isNaN(trimmedResult) ? 0 : trimmedResult;      return parseFloat(finalResult); } 

For those who need speed but also want to avoid floating-point errors, try something like BigDecimal.js. You can find other javascript BigDecimal libraries in this SO question: "Is there a good Javascript BigDecimal library?" and here's a good blog post about math libraries for Javascript