JavaScript: Convert String to Number

There are two main ways to convert a string to a number in javascript. One way is to parse it and the other way is to change its type to a Number. All of the tricks in the other answers (e.g. unary plus) involve implicitly coercing the type of the string to a number. You can also do the same thing explicitly with the Number function.

1. Parsing

parseInt and parseFloat are the two functions used for parsing strings to numbers. Parsing will stop silently if it hits a character it doesn’t recognise, which can be useful for parsing strings like “92px”, but it’s also somewhat dangerous, since it won’t give you any kind of error on bad input, instead you’ll get back NaN unless the string starts with a number. Whitespace at the beginning of the string is ignored. Here’s an example of it doing something different to what you want, and giving no indication that anything went wrong:

It’s good practice to always specify the radix as the second argument. In older browsers, if the string started with a 0, it would be interpreted as octal if the radix wasn’t specified which took a lot of people by surprise. The behaviour for hexadecimal is triggered by having the string start with 0x if no radix is specified, e.g. 0xff. The standard actually changed with ecmascript 5, so modern browsers no longer trigger octal when there’s a leading 0 if no radix has been specified. parseInt understands radixes up to base 36, in which case both upper and lower case letters are treated as equivalent.

2. Changing the Type of a String to a Number

All of the other tricks mentioned above that don’t use parseInt, involve implicitly coercing the string into a number. I prefer to do this explicitly,

This has different behavior to the parse methods (although it still ignores whitespace). It’s more strict: if it doesn’t understand the whole of the string than it returns NaN, so you can’t use it for strings like 97px. Since you want a primitive number rather than a Number wrapper object, make sure you don’t put new in front of the Number function.

Obviously, converting to a Number gives you a value that might be a float rather than an integer, so if you want an integer, you need to modify it. There are a few ways of doing this:

Any bitwise operator (here I have done a bitwise or, but you could also do double negation as in an earlier answer or a bitshift) will convert the value to a 32bit integer, and most of them will convert to a signed integer. Note that this will not do want you want for large integers. If the integer cannot be represented in 32bits, it will wrap.

To work correctly with larger numbers, you should use the rounding methods

Bear in mind that all of these methods understand exponential notation, so 2e2 is 200 rather than NaN. Also, Number understands “Infinity”, while the parse methods don’t.

3. Custom

It’s unlikely that either of these methods do exactly what you want. For example, usually I would want an error thrown if parsing fails, and I don’t need support for Infinity, exponentials or leading whitespace. Depending on your usecase, sometimes it makes sense to write a custom conversion function.

Always check that the output of Number or one of the parse methods is the sort of number you expect. You will almost certainly want to use isNaN to make sure the number is not NaN (usually the only way you find out that the parse failed).

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