How do I convert a float number to a whole number in JavaScript?

To convert a floating-point number to a whole number in JavaScript, you can use several methods depending on whether you want to round, truncate, or floor/ceil the value. Here’s a breakdown of common approaches:

1. Truncate Decimals (Remove Fraction)

Math.trunc()

Removes the decimal part, leaving the integer (works for positive and negative numbers):

Math.trunc(3.7);   // 3
Math.trunc(-3.7);  // -3

Bitwise Operators

Truncates by converting to a 32-bit integer (fast but limited to 32-bit range):

3.7 | 0;    // 3
-3.7 | 0;   // -3
~~3.7;      // 3 (double bitwise NOT)

2. Round to Nearest Integer

Math.round()

Rounds to the nearest whole number:

Math.round(3.2);   // 3
Math.round(3.5);   // 4 (midpoint rounds up)
Math.round(-3.7);  // -4

3. Round Down (Math.floor) or Up (Math.ceil)

Math.floor()

Rounds down to the nearest integer:

Math.floor(3.7);   // 3
Math.floor(-3.7);  // -4

Math.ceil()

Rounds up to the nearest integer:

Math.ceil(3.2);    // 4
Math.ceil(-3.7);   // -3

4. Parse as Integer

parseInt()

Truncates decimals (works for strings and numbers):

parseInt(3.7);     // 3
parseInt("4.9");   // 4 (works with strings)

5. Using toFixed(0) (String Conversion)

Converts to a string and back to a number (rounds to nearest integer):

parseFloat((3.7).toFixed(0));   // 4
parseFloat((-3.7).toFixed(0));  // -4

Key Differences

Method	3.7	-3.7	3.2	3.5	Notes
Math.trunc()	3	-3	3	3	Removes decimals.
Math.round()	4	-4	3	4	Rounds to nearest integer.
Math.floor()	3	-4	3	3	Rounds down.
Math.ceil()	4	-3	4	4	Rounds up.
~~num or | 0	3	-3	3	3	Truncates (32-bit limit).
parseInt(num)	3	-3	3	3	Truncates decimals.

When to Use Which

• Truncate: Use Math.trunc() or bitwise operators (~~ or | 0).
• Round: Use Math.round(), Math.floor(), or Math.ceil() based on direction.
• Edge Cases: Bitwise operators (~~, | 0) fail for numbers outside 32-bit range (e.g., 2147483648.5 → -2147483648).
• String Handling: Use parseInt() if working with string inputs.

Example Workflow

const floatNum = 4.9;

// Truncate to integer
const truncated = Math.trunc(floatNum); // 4

// Round to nearest integer
const rounded = Math.round(floatNum);   // 5

// Parse as integer (truncate)
const parsed = parseInt(floatNum);      // 4

// Bitwise truncation
const bitwiseTrunc = floatNum | 0;     // 4

Choose the method that best matches your use case!