Example 1 of the knockout extenders page describes a way of rounding user input and making sure it is only numeric.

It works great, but looking through the source they do a peculiar thing that i don't understand, that is on line 8 they do this:

parseFloat(+newValue)

newValue is a string.

When i initially asked this question I didn't know what + did - some further poking and a link to a different MDN page from one of the initial answers I got indicate it is a unary operator equivalent to number(str) and that there are some differences between +str and parseFloat(str) (treatment of strings ending in alpha characters and interpretation of hex seem to be the headlines).

I still don't understand why the + in this case needed to be wrapped in the parseFloat although I am starting to think it might be a typo...

Example 1 of the knockout extenders page describes a way of rounding user input and making sure it is only numeric.

It works great, but looking through the source they do a peculiar thing that i don't understand, that is on line 8 they do this:

parseFloat(+newValue)

newValue is a string.

When i initially asked this question I didn't know what + did - some further poking and a link to a different MDN page from one of the initial answers I got indicate it is a unary operator equivalent to number(str) and that there are some differences between +str and parseFloat(str) (treatment of strings ending in alpha characters and interpretation of hex seem to be the headlines).

I still don't understand why the + in this case needed to be wrapped in the parseFloat although I am starting to think it might be a typo...

• javascript

• 1 I just found this stackoverflow./questions/12227594/… question which is similar - the answerers explanation of the differences between parseFloat and + are pretty good. But it still doesn't explain why the number should be parsed afterwards... – jaybeeuu Commented Oct 28, 2015 at 15:26
• It simply makes no sense to use both. Calling parseFloat on a number is an antipattern. – Bergi Commented Oct 28, 2015 at 15:30
• Can you limit your question to #2? – Bergi Commented Oct 28, 2015 at 15:30
• Thanks Bergi - I don't think i makes any sense either... – jaybeeuu Commented Oct 28, 2015 at 15:43
• The line in question seems equivalent to +newValue || 0 – jaybeeuu Commented Oct 28, 2015 at 16:05

1 Answer 1

Citing MDN docs for parseFloat:

parseFloat parses its argument, a string, and returns a floating point number. If it encounters a character other than a sign (+ or -), numeral (0-9), a decimal point, or an exponent, it returns the value up to that point and ignores that character and all succeeding characters. Leading and trailing spaces are allowed.

parseFloat('0.32abcd') // -> 0.32
parseFloat(+'0.32abcd') // -> NaN

After a bit of digging in docs and running some tests, seems there is no reason to use parseFloat other than parsing strings that may contain numbers with non numeric trails to number, eq:

parseFloat('31.5 miles') // -> 31.5
parseFloat('12.75em') // -> 12.75

For any other cases where your string contains number + is a fastest and prefered way (citing MDN docs for unary plus operator):

unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number.

See parseFloat versus unary test case for how faster it is.

Previous link broken so here is the new test that shows how unary is faster.