Which is better, number(x) or parseFloat(x)?

Which is better?

I'm asking this just for the sake of shaving a few bytes, as I can use +x instead of number(x). Does parsefloat do something better?

The difference between parseFloat and Number

parseFloat/parseInt is for parsing a string, while Number/+ is for coercing a value to a number. They behave differently. But first let's look at where they behave the same:

parseFloat('3'); // => 3
Number('3'); // => 3
parseFloat('1.501'); // => 1.501
Number('1.501'); // => 1.501
parseFloat('1e10'); // => 10000000000
Number('1e10'); // => 10000000000

So as long as you have standard numeric input, there's no difference. However, if your input starts with a number and then contains other characters, parseFloat truncates the number out of the string, while Number gives NaN (not a number):

parseFloat('1x'); // => 1
Number('1x'); // => NaN

In addition, Number understands hexadecimal input while parseFloat does not:

parseFloat('0x10'); // => 0
Number('0x10'); // => 16

But Number acts weird with empty strings or strings containing only white space:

parseFloat(''); // => NaN
Number(''); // => 0
parseFloat(' \r\n\t'); // => NaN
Number(' \r\n\t'); // => 0

On the whole, I find Number to be more reasonable, so I almost always use Number personally (and you'll find that a lot of the internal JavaScript functions use Number as well). If someone types '1x' I prefer to show an error rather than treat it as if they had typed '1'. The only time I really make an exception is when I am converting a style to a number, in which case parseFloat is helpful because styles come in a form like '3px', in which case I want to drop the 'px' part and just get the 3, so I find parseFloat helpful here. But really which one you choose is up to you and which forms of input you want to accept.

Note that using the unary + operator is exactly the same as using Number as a function:

Number('0x10'); // => 16
+'0x10'; // => 16
Number('10x'); // => NaN
+'10x'; // => NaN
Number('40'); // => 40
+'40'; // => 40

So I usually just use + for short. As long as you know what it does, I find it easy to read.

The difference is what happens when the input is not a "proper number". Number returns NaN while parseFloat parses "as much as it can". If called on the empty string Number returns 0 while parseFloat returns NaN.

For example:

Number("") === 0               // also holds for false
isNaN(parseFloat("")) === true // and null

isNaN(Number("32f")) === true
parseFloat("32f") === 32

In these examples you can see the difference:

Number('') = 0;
Number(false) = 0;
Number('1a') = NaN;

parseFloat('') = NaN;
parseFloat(false) = NaN;
parseFloat('1a') = 1;

parseFloat is a bit slower because it searches for first appearance of a number in a string, while the Number constuctor creates a new number instance from strings that contains numeric values with whitespace or that contains falsy values.