what is ['1','2','3'].map(parseInt) result

Question

['1','2','3'].map(parseInt)

return [1, NaN, NaN]

I don't know why? In my opinion is like this:

['1','2','3'].map(function(i){return parseInt(i,10)})

return [1, 2, 3]

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and other
['1','2','3'].map(parseFloat)
return [1, 2, 3]

look at the map callback arguments - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map — Arun P Johny, Feb 26 '14 at 09:01
Check the output of this: `['1', '2', '3'].map(function () { return arguments });` — fardjad, Feb 26 '14 at 09:03

score 10 · Answer 1 · edited May 23 '17 at 11:45

10

You can find answer from Mozilla developers site:

// Consider:
["1", "2", "3"].map(parseInt);
// While one could expect [1, 2, 3]
// The actual result is [1, NaN, NaN]

// parseInt is often used with one argument, but takes two. The second being the radix
// To the callback function, Array.prototype.map passes 3 arguments: 
// the element, the index, the array
// The third argument is ignored by parseInt, but not the second one,
// hence the possible confusion. See the blog post for more details


function returnInt(element){
  return parseInt(element,10);
}

["1", "2", "3"].map(returnInt);
// Actual result is an array of numbers (as expected)

Also read great answer

edited May 23 '17 at 11:45

Community

1
1

answered Feb 26 '14 at 09:01

Farkhat Mikhalko

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Your answer is great.But @Howard's easier to understand.So I chose his answer. Thank you very much for your answer. – lemon郑 Feb 26 '14 at 09:16
1

@lemon郑 no problem, i glad to help to you)) – Farkhat Mikhalko Feb 26 '14 at 09:17
This answer is outdated. You can now use `parseFloat` instead of creating a new function. – Sjieke Oct 26 '22 at 16:13

score 7 · Accepted Answer · answered Feb 26 '14 at 09:02

Check out this article: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

The callback function is specified as:

callback
Function that produces an element of the new Array, taking three arguments:
currentValue
The current element being processed in the array.
index
The index of the current element being processed in the array.
array
The array map was called upon.

Therefore your map() function expands into:

parseInt('1', 0, the_array) # 1
parseInt('2', 1, the_array) # NaN
parseInt('3', 2, the_array) # NaN

what is ['1','2','3'].map(parseInt) result

2 Answers2