What does the + in this statement do: a = +array[i]

Question

I'm looking at the source code of the sum function of d3 and I'm wondering what the + in line 8 and 10 does. It's this statement:

a = +array[i]

So it's an assignment of the ith element of array to a, but what does the + in front of array[i] do? The same syntax is repeated in line 10, where the + precedes the function call. I have never seen such a syntax in JavaScript.

Look up *unary plus*. It effectively causes [ToNumber] to be applied to the operand. — user2864740, Sep 09 '14 at 09:09
possible duplicate of [Whats the significant use of Unary Plus and Minus operators?](http://stackoverflow.com/questions/5450076/whats-the-significant-use-of-unary-plus-and-minus-operators) — user2864740, Sep 09 '14 at 09:10

score 1 · Answer 1 · answered Sep 09 '14 at 09:10

What you're really doing is applying the operator + to the element at index i in the array. And what does that operator do? Basically, it's a shorthand way of forcibly turning a variable into a number. As an example, run the following code:

+new Date()

Instead of a date object, you'll get an integer representation of the date.

What does the + in this statement do: a = +array[i]

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