Javascript map native object methods

In Python, you can something like this:

>>> list(map(str.upper, ['foo','bar']))
['FOO', 'BAR']

I would like to be able to do something similar in javascript:

I've tried t开发者_StackOverflowhe following in Chrome using the native map implementation ( https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/map ):

['foo','bar'].map(String.prototype.toUpperCase)

['foo','bar'].map(String.prototype.toUpperCase.call)

Why doesn't call work? Is there any elegant way to do this or must I wrap toUpperCase in a callback function? Thx

Try:

['foo','bar'].map(Function.prototype.call.bind(String.prototype.toUpperCase))

or for the same effect:

['foo','bar'].map(Function.call.bind("".toUpperCase))

JavaScript's treatment of this is a bad trip.

Modern browsers support this (although it is a recent addition)

var a = ['foo','bar'];
var b = a.map(String.toUpperCase);

alert(b[1]);

or even

var a = ['foo','bar'].map(String.toUpperCase);
alert(a[0]);

example: http://www.jsfiddle.net/xPsba/

That is because String.prototype.toUpperCase runs on this (Context) object but map function can't set the array element as context. It passes it as an argument. A workaround would be

['foo','bar'].map(function(k) { return String.prototype.toUpperCase.call(k) });

You're all missing the point. :-)

We want to call a method (not a function) on all elements of an array. You can use map:

a.map(function(x){x.frob()})

but that's too much typing. We want to say:

a.mapm(A.frob)

where A is the class of the elements of a and mapm is a new Array method that calls its argument method on each element of this (array a in this case). You could define mapm thus:

Array.prototype.mapm = function (method)
{
    return this.map(function (x) { return method.apply(x) } )
};

and call it thus:

["a", "b", "c"].mapm("".toUpperCase)    ==>     ["A", "B", "C"]

The only problem is that you've added a new element, mapm to every array, though it's ignored by most Array methods, e.g. length and map.

You can use something like this:

function map(arr, fn){
    var i = 0, len = arr.length, ret = [];
    while(i < len){
        ret[i] = fn(arr[i++]);
    }
    return ret;
}

And to use it:

var result = map(['foo', 'bar'], function(item) {
    return item.toUpperCase();
});

console.log ( result ); // => ['FOO', 'BAR']