I don't understand this map() behavior

Question

Hi JavaScript masters !

console.log([ '5', '9', '7' ].map(parseInt)) tells me [5, NaN, NaN]

console.log([ '5', '9', '7' ].map(function(element){
    return parseInt(element)
}))

tells me [5, 9, 7]

But why ? ^^'

`console.log([ '5', '9', '7' ].map(Number))` tells me [5, 9, 7] ... — Olivier Meunier, Oct 08 '19 at 12:22
Thank you @Jiří the answer is there : https://stackoverflow.com/questions/262427/why-does-parseint-yield-nan-with-arraymap — Olivier Meunier, Oct 08 '19 at 12:25
[Tricky use case](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map#Tricky_use_case) — crashmstr, Oct 08 '19 at 12:26
should be `console.log([ '5', '9', '7' ].map(x => parseInt(x)));` — doğukan, Oct 08 '19 at 12:26
If you read the MDN documentation, *every* example until the "Tricky use case" that is this exact problem is using an anonymous function to do the work. — crashmstr, Oct 08 '19 at 12:28

score 2 · Answer 1 · edited Jun 20 '20 at 09:12

2

This happens because, as it is described here:

parseInt is often used with one argument, but takes two. The first is an expression and the second is 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.

edited Jun 20 '20 at 09:12

Community

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answered Oct 08 '19 at 12:21

Christos

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score 0 · Answer 2 · answered Oct 08 '19 at 12:22

map calls the function you give it with 3 parameters: the value, the index of it in the array, and the array itself. Your second approach safely ignores these, but your first one causes the second parameter to be interpreted as the base of the number, which isn't what you wanted.

score 0 · Answer 3 · answered Oct 08 '19 at 12:25

0

Best explained here: https://stackoverflow.com/a/262511/6935763

No use of copying pasting here the whole answer

answered Oct 08 '19 at 12:25

Abhay Maurya

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Posting a link isn't very useful either. Just cast a duplicate vote. – georg Oct 08 '19 at 12:31
um...i did? You can see my name there who marked it duplicated! I just posted as an answer for anyone who scrolls down looking for answer here anyway – Abhay Maurya Oct 08 '19 at 12:38

I don't understand this map() behavior

3 Answers3