The parseInt Function in JavaScript

Apr 4, 2023

JavaScript has a built-in parseInt() function that parses a string into a number. The parseInt() function takes 2 parameters: the string to parse, and the radix. You should set the radix parameter to 10, unless you are positive you need to set it to something else.

parseInt('42', 10); // 42

// `parseInt()` also ignores leading and trailing whitespace
parseInt('  42', 10); // 42
parseInt('42  ', 10); // 42

The parseInt() function ignores any non-numeric characters after the number, and ignores leading whitespace. It also handles numbers that start with +.

parseInt('42abc', 10); // 42
parseInt('42,77', 10); // 42
parseInt('+42', 10); // 42
parseInt('-42', 10); // -42

Error Handling

If you pass in a string that doesn't contain a valid number, parseInt() will return NaN. parseInt() will not throw an error.

parseInt('a42', 10); // NaN


The radix parameter represents the numeric base, like 2 for binary or 16 for hexadecimal. Mastering JS uses the radix parameter for converting binary to decimal and decimal to binary For example, you can also use parseInt() to parse binary or hexadecimal strings as follows.

parseInt('10110', 2); // 22
parseInt('FA', 16); // 250

The reason why you should always explicitly set the radix to 10 is because parseInt() assumes strings that starts with 0x or 0X should be parsed as hexadecimal unless radix is explicitly set.

parseInt('0xFF'); // 255, radix set to 16 implicitly
parseInt('0xFF', 10); // 0

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