Strange behavior for Map, parseInt [duplicate]

Possible Duplicate:
javascript - Array.map and parseInt

I saw this example of strange JavaScript behavior on twitter

['10','10','10','10','10'].map(parseInt)

evaluates to

[10, NaN, 2, 3, 4]

could somebody explain this behavior? I verified it in chrome and firebug

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

correctly returns an array of 10s as integers. Is this an improper use of map(), a bug with parseInt, or something else?

Solution 1:

parseInt receives two arguments: string and radix:

var intValue = parseInt(string[, radix]);

while map handler's second argument is index:

... callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

parseInt uses the first two arguments being passed in by map, and uses the second argument to specify the radix.

Here's what's happening in your code:

parseInt('10', 0) // 10
parseInt('10', 1) // NaN
parseInt('10', 2) // 2
parseInt('10', 3) // 3
parseInt('10', 4) // 4

Here's a link to MDN on parseInt: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/parseInt.

From MDN:

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

parseInt() takes two arguments. The value and the radix. That means that the parseInt() function is being called with unintended parameters.