How to truncate decimals in JavaScript without Math library?

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I need numbers to have only 2 decimals (as in money), and I was using this:

Number(parseFloat(Math.trunc(amount_to_truncate * 100) / 100));

But I can no longer support the Math library.

How can I achieve this without the Math library AND withou rounding the decimals?

Make it simple

const trunc = (n, decimalPlaces) => {
  const decimals = decimalPlaces ? decimalPlaces : 2;
  const asString = n.toString();
  const pos = asString.indexOf('.') != -1 ? asString.indexOf('.') + decimals + 1 : asString.length;
  return parseFloat(n.toString().substring(0, pos));
};

console.log(trunc(3.14159265359));
console.log(trunc(11.1111111));
console.log(trunc(3));
console.log(trunc(11));
console.log(trunc(3.1));
console.log(trunc(11.1));
console.log(trunc(3.14));
console.log(trunc(11.11));
console.log(trunc(3.141));
console.log(trunc(11.111));

Rounding and truncating numbers in JavaScript, There are several built-in functions for rounding: Removes anything after the decimal point without rounding: 3.1 becomes 3 , -1.1 becomes -1 . This rounds up or down to the nearest value, similar to Math.round : let num� Returns Decimal. The integral part of d; that is, the number that remains after any fractional digits have been discarded.. Examples. The following example calls the Truncate(Decimal) method to truncate both a positive and a negative Decimal value.

You can use toFixed

Number(amount_to_truncate.toFixed(2))

If you are sure that your input always will be lower or equal than 21474836.47 ((2^31 - 1) / 100) (32bit) then:

if you need as string (to make sure result will have 2 decimals)

((amount_to_truncate * 100|0)/100).toFixed(2)

Otherwise

((amount_to_truncate * 100|0)/100)

Else: See Nina Schols's answer

console.log((((15.555 * 100)|0)/100)) // will not round: 15.55
console.log((((15 * 100)|0)/100).toFixed(2)) // will not round: 15.55

Numbers, You might be tempted to reach for an external library like math.js, whose math. round function takes the number of decimals as an argument, Floating point math in JavaScript can produce rounding errors at times, as I On Medium, smart voices and original ideas take center stage - with no ads in sight. Definition and Usage. The toFixed() method converts a number into a string, rounding to a specified number of decimals. Note: if the desired number of decimals are higher than the actual number, zeros are added to create the desired decimal length.

You could use parseInt for a non rounded number.

console.log(parseInt(15.555 * 100, 10) / 100); // 15.55 no rounding
console.log((15.555 * 100 | 0) / 100);         // 15.55 no rounding, 32 bit only 
console.log((15.555).toFixed(2));              // 15.56 rounding

How to Round to a Certain Number of Decimal Places in JavaScript , Use Math.trunc() to truncate a floating point number and return its integer part. doesn't do any rounding, it simply removes all the digits following the decimal. If you're curious, I wrote up a performance test comparing these two functions. Definition and Usage. The trunc() method returns the integer part of a number. Note: This method will NOT round the number up/down to the nearest integer, but simply remove the decimals.

The only thing I see wrong with toFixed is that it rounds the precision which OP specifically states they don't want to do. Truncate is more equivalent to floor for positive numbers and ceil for negative than round or toFixed. On the MDN page for the Math.trunc there is a polyfill replacement function that would do what OP is expecting.

Math.trunc = Math.trunc || function(x) {
  return x - x % 1;
}

If you just used that, then the code wouldn't have to change.

Math.trunc(), Rounding means replacing a number with an approximate value that has a shorter, simpler, Accurate rounding of transcendental mathematical functions is difficult 0 and x (included); i.e. y is the integer part of x , without its fraction digits . the computed digits with a limited precision (notably within standard JavaScript or� Rounding numbers with decimal precision requires a little bit of calculation and Math.round(). Optionally we can use the toFixed() method that belongs to the Number prototype. The output type of toFixed() is a string which needs to be passed to a top-level function called parseFloat() to return a number .

Try using toFixed:

number.toFixed(2)

Number Truncation in JavaScript, Rounding numbers with JavaScript to decimals and to integers. To round up to the next higher integer, use the Math.ceil() function. To round down to As indicated, the Math.ceil() , Math.floor() , and Math.round() functions can be used for quick rounding to an integer. Importing Content for WordPress Without Plugins� Questions: 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. Answers: upd: So, after all it turned out, rounding

Rounding, Example. Round a number to the nearest integer: Math.round(2.5);. Try it Yourself �. More "Try it Yourself" examples below. Unlike the other three Math methods: Math.floor(), Math.ceil() and Math.round(), the way Math.trunc() works is very simple. 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.

JavaScript Rounding, Output: 15. The Math.trunc() method is used to return the integer part of a floating- point number by removing the fractional digits. In other words� A number can be rounded off to upto 2 decimal places using two different approaches in javascript. Method 1: Using toFixed() method. The Number.toFixed() method takes an integer as an input and returns the the given number as a string in which the fraction is padded to the input integers length.

JavaScript round() Method, Well organized and easy to understand Web building tutorials with lots of examples of how to use HTML, CSS, JavaScript, SQL, PHP, Python, Bootstrap, Java and XML.

Comments
  • Why can't you use Math? It's available everywhere.
  • Number(amount_to_truncate.toFixed(2))
  • I just find out it fails on trunc(2.9999999999999999999999) = 3. I really hate JavaScript.
  • The OP wants to truncate the result. toFixed will round instead.
  • @MichaelLiu answer is updated. I still insist with toFixed
  • ((2.28 * 100|0)/100).toFixed(2) outputs "2.27" because console.log(2.28 * 100); outputs 227.99999999999997. Number(2.286.toFixed(2)) outputs 2.29 because toFixed rounds @TolgahanAlbayrak
  • This yields a surprising result for large numbers like 1000000000000000000000. It may be safer to avoid using parseInt.
  • the value is unlikely for money, as OP mentioned.
  • console.log((15 * 100 | 0) / 100); will output 15 instead of 15.00
  • @TolgahanAlbayrak, the result, you mentioned, is only possible with a string.
  • welcome to the wonderful world of floating point arithmetic.
  • You are probably right, but I can't (or shouldn't) add additional libraries to this particular project. Looks like manonthemat answer is the only one that works so far.
  • @DavidPrieto: THX for your feedback, as i'm also truly interested in this topic, could you please kindly explain me where my round2() function fails against your accepted trunc() function? It's just onlly because you don't need the Math library or do you have found some other issues?
  • I'm seeing your edit just now. It looks like both functions work basically the same way, and I just tested that they both fail in round2(2.9999999999999999999999) = 3