# Why does parseInt yield NaN with Array#map?

From the Mozilla Developer Network:

``````[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).

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]
``````

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."

So if you call a function `parseInt` 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 based on the input (base 10, in this case). Base 1 is an impossible number base, and 3 is not a valid number in base 2:

``````parseInt('1', 0); // OK - gives 1
parseInt('2', 1); // FAIL - 1 isn't a legal radix
parseInt('3', 2); // FAIL - 3 isn't legal in base 2
``````

So in this case, you need the wrapper function:

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

or with ES2015+ syntax:

``````['1','2','3'].map(num => parseInt(num, 10));
``````

(In both cases, it's best to explicitly supply a radix to `parseInt` as shown, because otherwise it guesses the radix based on the input. In some older browsers, a leading 0 caused it to guess octal, which tended to be problematic. It will still guess hex if the string starts with `0x`.)

`map` is passing along a 2nd argument, which is (in many of the cases) messing up `parseInt`'s radix parameter.

If you're using underscore you can do:

`['10','1','100'].map(_.partial(parseInt, _, 10))`

Or without underscore:

`['10','1','100'].map(function(x) { return parseInt(x, 10); });`

``````var a = ['0', '1', '2', '10', '15', '57'].map(Number);
``````console.log(parseInt("19asdf"));