Multiple Variables Javascript map() callback

Question

I was doing some reading about es2015, trying out some of the examples on the arrow syntax when I came across this code:

var odds = evens.map(v => v + 1);//I understand
var nums = evens.map((v,i) => v + i);//I don't understand

with evens initialized to [0,2,4,6] I get [1,3,5,7] from odds, which I understand.

However I don't understand the result of the the second expression [0,3,6,9]. Essentially I'm not understanding what values are being assigned to the variable i in the second example. Can anyone shed any light on this?

Read the documentation for Array#map, in particular the part about what parameters it passes to the callback. — user663031
– user663031, Commented Dec 15, 2015 at 4:39

thefourtheye · Accepted Answer · 2015-12-15 02:17:22Z

6

Array.prototype.map function, will call the function passed to it with three parameters, as seen here

current element
current element's index and
the actual array itself

So, in your case, v is the current element and i is the index of the element in the array.

When evens is initialized with [0, 2, 4, 6], it just adds the element with its corresponding index and returns a new array.

answered Dec 15, 2015 at 2:17

thefourtheye

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