javscript - Array.map and parseInt

Question

From the Mozilla Dev Site:

[1,4,9].map(Math.sqrt)

will yield:

[1,2,3]

Why then does this:

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

yield this:

[1, NaN, NaN]

I have tested in Firefox 3.0.1 and Chrome 0.3 and just as a disclaimer, I know this is not cross-browser functionality. (No IE)

[edit] I found out that the following will accomplish the desired effect. However, it still doesn't explain the errant behavior of parseInt.

['1','2','3'].map(function(i){return +i;}) // returns [1,2,3]

Answer 1

I'm going to wager that it's something funky going on with the parseInt's 2nd parameter, the radix. Why it is breaking with the use of Array.map and not when you call it directly, I do not know.

//  Works fine
parseInt( 4 );
parseInt( 9 );

//  Breaks!  Why?
[1,4,9].map( parseInt );

//  Fixes the problem
[1,4,9].map( function( num ){ return parseInt( num, 10 ) } );

Answer 2

Try this:

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

parseInt() is a function.

m =  ['1','2','3'].map(parseInt())
alert(m[2]) >> 3

Answer 3

The callback function in Array.map has three parameters:

From the same Mozilla page that you linked to: "callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed." (https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/Array/map)

So if you call a function which actually expects two arguments, the second argument will be the index of the element.

In this case, you ended up calling parseInt with radix 0, 1 and 2 in turn. The first is the same as not supplying the parameter, so it defaulted to base 10. Base 1 is an impossible number base, and 3 is not a valid number in base 2.