ParseInt vs Number in JavaScript: Key Differences and Usage

parseInt converts a string to an integer by reading digits until a non-digit appears, ignoring decimals, while Number converts the entire string to a number including decimals and returns NaN if invalid. Use parseInt for integer parsing and Number for full numeric conversion.

Quick Comparison

This table summarizes the main differences between parseInt and Number in JavaScript.

Feature	parseInt	Number
Type of conversion	Parses integer from string, stops at first non-digit	Converts whole string to number (integer or float)
Handles decimals	Ignores decimals, returns integer only	Parses decimals correctly
Returns on invalid input	Returns parsed integer up to invalid character or NaN if none	Returns NaN if entire string is invalid
Input types	Usually string, ignores trailing characters after digits	Converts string, boolean, null, or other types to number
Radix support	Supports optional radix parameter to specify base	No radix parameter, always base 10 for strings
Whitespace handling	Ignores leading whitespace	Ignores leading and trailing whitespace

Key Differences

parseInt reads a string from left to right and converts as many numeric characters as possible into an integer. It stops parsing when it encounters a non-numeric character, so it can parse strings like "123abc" as 123. It also accepts a second argument called radix to specify the number base (like 10 for decimal or 16 for hexadecimal).

On the other hand, Number tries to convert the entire string into a number, including decimals. If the string contains any invalid characters or is not a valid number format, it returns NaN. It does not accept a radix parameter and treats strings as base 10 by default.

Additionally, Number can convert other types like booleans (true to 1, false to 0) and null to 0, while parseInt is mainly used for strings. This makes Number more versatile but stricter in parsing full numeric values.

Code Comparison

javascript

console.log(parseInt('123.45'));      // 123
console.log(parseInt('123abc'));      // 123
console.log(parseInt('abc123'));      // NaN
console.log(parseInt('  42  '));      // 42
console.log(parseInt('0xF', 16));     // 15

Output

123
123
NaN
42
15

Number Equivalent

javascript

console.log(Number('123.45'));        // 123.45
console.log(Number('123abc'));        // NaN
console.log(Number('abc123'));        // NaN
console.log(Number('  42  '));        // 42
console.log(Number('0xF'));            // 15
console.log(Number(true));             // 1
console.log(Number(false));            // 0
console.log(Number(null));             // 0

Output

123.45
NaN
NaN
42
15
1
0
0

When to Use Which

Choose parseInt when you need to extract an integer from the start of a string, especially if the string may contain extra characters after the number or if you want to specify a number base (radix).

Choose Number when you want to convert a whole string or other types into a number, including decimals, and want strict validation that the entire input is a valid number.

In summary, use parseInt for flexible integer parsing and Number for precise full numeric conversion.

Key Takeaways

parseInt parses integers from strings and stops at the first invalid character.

Number converts entire strings or values to numbers, including decimals, or returns NaN if invalid.

parseInt supports a radix parameter; Number does not.

Use parseInt for integer extraction and Number for full numeric conversion.

Number can convert booleans and null, while parseInt is mainly for strings.