Get the Decimal Part of a Number in JavaScript

Get the Decimal Part of a Number #

To get the decimal part of a number:

1. Check if the number is an integer, if it is, return 0.
2. Otherwise, convert the number to a string and use the split() method to split it on the dot.
3. Convert the array element at index 1 to a number and return the result.

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const num = 12.345;

function getDecimalPart(num) {
  if (Number.isInteger(num)) {
    return 0;
  }

  const decimalStr = num.toString().split('.')[1];
  return Number(decimalStr);
}

console.log(getDecimalPart(12.345)); // 👉️ 345
console.log(getDecimalPart(-1.23)); // 👉️ 23
console.log(getDecimalPart(1)); // 👉️ 0

We created a reusable function that returns the decimal part of a number.

We used the Number.isInteger method to determine if the passed in number is an integer.

If the number is an integer, we return 0 straight away as it has no decimal part.

Here are some examples of using the Number.isInteger method.

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console.log(Number.isInteger(5)); // 👉️ true
console.log(Number.isInteger(-5)); // 👉️ true
console.log(Number.isInteger(5.0)); // 👉️ true
console.log(Number.isInteger(5.5)); // 👉️ false
console.log(Number.isInteger('5')); // 👉️ false

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console.log(5.00 === 5); // 👉️ true

After we find out that the number isn't an integer, we convert it to a string and call the String.split method on it to split it on the dot.

The split method takes a separator as a parameter and splits the string into an array of substrings.

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console.log('1.37'.split('.')); // 👉️ ['1', '37']
console.log('-3.58'.split('.')); // 👉️ ['-3', '58']

The decimal part of the number is stored in the array at index 1. However, we have to convert it back to a number before returning from the function.

An alternative approach is to use the modulo operator.

To get the decimal part of a number use the modulo (%) operator to get the remainder from dividing the number by 1, e.g. 3.17 % 1. The modulo operator returns the remainder of dividing one number by another.

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function getDecimalPart(num) {
  return num % 1;
}

console.log(getDecimalPart(3.137)); // 👉️ 0.137
console.log(getDecimalPart(1.37)); // 👉️ 0.37000000000001
console.log(getDecimalPart(5)); // 👉️ 0

We used the modulo (%) operator to get the remainder of dividing the number by 1.

This gives us the number after the decimal, however floating point numbers don't represent all decimals precisely in binary, which sometimes leads to inconsistent results.

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console.log(0.1 + 0.2 === 0.3); // 👉️ false

The sum of 0.1 and 0.2 is actually equal to 0.30000000000000004 instead of 0.3. This is because the binary floating-point format cannot accurately represent numbers like 0.1 or 0.2.

The code gets rounded to the nearest number, resulting in a rounding error.

Further Reading #

• Get the value Before the Decimal Point in JavaScript
• Get the number of digits before the Decimal in JavaScript