Single plus operator in javascript

Question

Just saw this in underscore's source:

if (obj.length === +obj.length) {
    ...
}

What does the plus do? I never saw this before.

Is it considered a good practice among developers?

@epascarello The point of this question is also if the named technique is considered a good practice among developers, which is not discused on the other question — jviotti, Jan 23 '13 at 01:31
Good Practice is a "I think blue is best color" type of thing. One guy will say, no, use Number(), other guy will say use +. And the selected answer here does not say it. ;) — epascarello, Jan 23 '13 at 02:09

Joseph Silber · Accepted Answer · 2013-01-23T01:41:28.843

35

The plus converts a string to a float. The code you provided is equivalent to the following:

if ( obj.length === Number(obj.length) ) {
    // ...
}

edited Jan 23 '13 at 01:41

answered Jan 23 '13 at 01:22

Joseph Silber

4

It's also equivalent to `if(typeof obj.length === "number")`, which would be better as it's much less confusing. – Niet the Dark Absol Jan 23 '13 at 01:23
3

`if(var === !!var)` same trick with booleans – Peter Jan 23 '13 at 01:25
1

Is this considered a good practice? – jviotti Jan 23 '13 at 01:33
@jviotti - I think so. I use it all the time. – Joseph Silber Jan 23 '13 at 01:35
5

This is equivalent to `Number(...)`, not `parseFloat(...)`, as easily shown by the input case "0xf". – PleaseStand Jan 23 '13 at 01:38
1

@PleaseStand - Thanks. Mea culpa. – Joseph Silber Jan 23 '13 at 01:41

1 Answers1