JavaScript How to Convert Decimal to Octal Number

Use number.toString(8) in JavaScript to convert a decimal number to its octal string representation, for example, (10).toString(8) returns '12'.

Examples

Input8

Output'10'

Input64

Output'100'

Input0

Output'0'

How to Think About It

To convert decimal to octal, think of changing the base from 10 to 8. JavaScript lets you do this easily by converting the number to a string in base 8 using toString(8). This gives the octal form as a string.

Algorithm

1

Get the decimal number input.

2

Use the number's <code>toString</code> method with 8 as the argument.

3

Return the resulting string as the octal representation.

Code

javascript

const decimalNumber = 83;
const octalString = decimalNumber.toString(8);
console.log(octalString);

Output

123

Dry Run

Let's trace converting decimal 83 to octal using toString(8).

1

Start with decimal number

decimalNumber = 83

2

Convert to octal string

octalString = 83..toString(8) => '123'

3

Print result

console.log(octalString) outputs '123'

Decimal	Octal String
83	123

Why This Works

Step 1: Using toString with base 8

The toString(8) method converts the decimal number into a string representing the number in base 8 (octal).

Step 2: Result is a string

The output is a string because octal numbers are usually represented as strings in programming.

Step 3: Works for any decimal number

This method works for any positive integer decimal number to convert it to octal.

Alternative Approaches

Manual division and remainder

javascript

function decimalToOctal(num) {
  let octal = '';
  while (num > 0) {
    octal = (num % 8) + octal;
    num = Math.floor(num / 8);
  }
  return octal || '0';
}
console.log(decimalToOctal(83));

This method shows the manual process of dividing by 8 and collecting remainders but is longer and less concise.

Using parseInt with radix

javascript

const decimalNumber = 83;
const octalString = parseInt(decimalNumber, 10).toString(8);
console.log(octalString);

This is similar to the main method but explicitly parses the number first; useful if input is a string.

Complexity: O(log n) time, O(log n) space

Time Complexity

Converting a number to a string in base 8 takes time proportional to the number of digits in octal, which is about log base 8 of the number.

Space Complexity

The space needed is proportional to the length of the octal string, also about log base 8 of the number.

Which Approach is Fastest?

Using toString(8) is the fastest and simplest built-in method compared to manual division.

Approach	Time	Space	Best For
toString(8)	O(log n)	O(log n)	Quick and simple conversion
Manual division	O(log n)	O(log n)	Learning how base conversion works
parseInt + toString	O(log n)	O(log n)	When input is string and needs parsing