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How to Round Numbers in JavaScript

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JavaScript has multiple ways to round a number. Some choices are the Math.round , number.toFixed , andnumber.toPrecision . You can also write your own function to round a number up or down to the nearest increment.

Math.round

Math.round rounds a number to the nearest integer. If the decimal part of the number is less than 0.5, it is rounded down. Otherwise, if the decimal part of the number of 0.5 or higher then it will be rounded up. The function returns the rounded number as the value.

For example:

Math.round(12.5); // 13  
Math.round(12.49); // 12

Number.toFixed

You can set the number of digits that appears after the decimal place with toFixed function. The function returns the string representation of the number as the value. It can be used like this:

const a = 12.8888888888;  
const b = a.toFixed(2); // 12.88

Number.toPrecision

Number.toPrecision is similar to toFixed . It returns the string representation of a number, but you can round it to the specified number of significant digits which you can specify or let it automatically round to the correct number of significant digits.

const a = 12.8888888888;  
const b = a.toPrecision(2); // 13, since 2 significant digits is specified  
const c = a.toPrecision(); // 12.8888888888, since all digits are significant in the original number

Round to the nearest Increment

You can round to the nearest increment up or down you specify:

const roundNumberUp = (num, increment) => {   
  return Math.ceil(num / increment) \* increment;  
}  
console.log(roundNumberUp(12.2, 0.5)) // 12.5

What this does is take the original number, divide by the increment you want to round up to, then take the ceiling of that, then multiply by the increment. This means the number should always round up.

Similarly, you can round down to the nearest increment with floor.

const roundNumberUp = (num, increment) => {   
  return Math.floor(num / increment) \* increment;  
}  
console.log(roundNumberUp(12.2, 0.5)) // 12.5

What this does is take the original number, divide by the increment you want to round up to, then take the floor of that, then multiply by the increment. This means the number should always round down.

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