CODEWITHSUNDEEP
javascriptnode.js
codeBarer
codeBarer

Reputation: 2398

Is JavaScript's array map prototype a little off?

I was playing around with JavaScript's array prototype map and I don't understand why I'm getting the following values: 

console.log(
    [1,2,2,5,6].map(function(a,b){ 
        return a+b; 
    })
); /*This returns [ 1, 3, 4, 8, 10 ]*/

Shouldn't the above code return [1,3,4,7,11] ?

Here's another snippet which shows that elements in the array are added sequentially and are correct at least I believe so.

console.log(
    [1,2,3,4,5,6].map(function(a,b){
        return a+b;
    })
); /*[ 1, 3, 5, 7, 9, 11 ]*/

This is just a curious question more along the lines of why the first snippet of code seems.

Upvotes: 1

Views: 56

Answers (3)

htoniv
htoniv

Reputation: 1668

so the first parameter 'a' is value and the 'b' is index. so adding together it shows the corect value only. so 'a' contains [1, 2, 2, 5, 6] and 'b' contains [0, 1, 2, 3, 4]

Upvotes: 1

Oriol
Oriol

Reputation: 288670

Let's sum:

  [1, 2, 2, 5, 6]  // Your values
+ [0, 1, 2, 3, 4]  // Indices
-----------------
  [1, 3, 4, 8,10]  // Result

  [1, 2, 3, 4, 5, 6]  // Your values
+ [0, 1, 2, 3, 4, 5]  // Indices
--------------------
  [1, 3, 5, 7, 9,11]  // Result

The results are correct.

I think you are confusing map with reduce:

var arr = [];
[1,2,2,5,6].reduce(function(a,b){ 
  arr.push(a+b);
  return b; 
}, 0);
arr; // [1, 3, 4, 7, 11]

  [0, 1, 2, 2, 5]  // Values returned in previous iteration
+ [1, 2, 2, 5, 6]  // Your values
-----------------
  [1, 3, 4, 7,11]  // Result

Upvotes: 4

Christopher Davies
Christopher Davies

Reputation: 4561

It's because map passes you the value as the first parameter and the index as the second. So, what you're adding is: 1 + 0, 2+1, 2+2, 5+3, etc.

Your a value is the value out of the array: 1, 2, 2, 5, 6

Your b value is actually an index, not a value out of the array: 0, 1, 2, 3, 4

Upvotes: 5

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