parseInt()

parseInt(string, radix)

The parseInt() function parses a string argument and returns an integer of the specified radix (base). It is commonly used to convert string representations of numbers into actual numeric values.

Syntax

parseInt(string)
parseInt(string, radix)

Parameters

Parameter	Type	Description
string	string	The value to parse. If not a string, it is converted to one first. Leading whitespace is ignored.
radix	number	An integer between 2 and 36 representing the base of the number system. Default is 10.

Return Value

The parsed integer. If the first character cannot be converted, NaN is returned.

Examples

Basic Integer Parsing

parseInt("42");
// 42

parseInt("  123  ");
// 123

parseInt("42px");
// 42 (parses until invalid character)

Parsing with Different Radices

// Binary (base 2)
parseInt("1010", 2);
// 10

// Octal (base 8)
parseInt("777", 8);
// 511

// Hexadecimal (base 16)
parseInt("ff", 16);
// 255

Common Pitfalls

// Leading zeros do not mean octal in modern JS (unless radix is 8)
parseInt("010");
// 10

// But with explicit radix 8:
parseInt("010", 8);
// 8

// Floating-point numbers are truncated
parseInt("3.14");
// 3

Invalid Input

parseInt("hello");
// NaN

parseInt("");
// NaN (unlike Number() which returns 0)

When to Use parseInt()

This function is useful when you need to:

• Convert user input strings to numbers
• Parse CSS pixel values (e.g., “100px”)
• Read numeric data from external sources that use specific radix formats

Differences from Number()

Unlike the Number() constructor, parseInt() has distinct behavior:

Number("42px");
// NaN (the entire string must be a valid number)

parseInt("42px");
// 42 (parses until it hits an invalid character)

Number("010");
// 10 (leading zeros are ignored)

parseInt("010");
// 10 (same, but radix matters)

Radix Best Practices

Always specify the radix when parsing:

// Explicit radix prevents unexpected behavior
parseInt("10", 10);
// 10

// Without radix, some implementations may guess incorrectly
parseInt("010");
// 10 (but could be 8 in older environments)

See Also

• JSON.parse() — Parse JSON strings into JavaScript values
• Number.isNaN() — Check if a value is NaN with stricter semantics
• eval() — Evaluate JavaScript code from a string