Is the javascript .map() function supported in IE8

问题:

When doing this:

var a = new Array("a", "b");
a.map(function() { });

in IE8 I get:

"Object doesn't support this property or method"

Is this method not supported in IE8, or do I have some other problem? I've had a Google, but get lots of Google Maps javascript issues/questions...

Edit: OK so IE8 and below DO NOT support the .map() function. Copy-paste the code from MDN here which will add the .map() function to the Array prototype exactly per the specs if not natively supported (and it seems to work perfectly).

回答1:

IE8 doesn't support map(). When in doubt, check MDN (Mozilla Developer Network):

map - MDN

Looks like IE added support for map() in version 9.

回答2:

The solution is jQuery.map

Instead of this: a.map(function( ) { });

You have to do

jQuery.map(a, function( ) { //what ever you want todo .. }

回答3:

(function(fn){
    if (!fn.map) fn.map=function(f){var r=[];for(var i=0;i<this.length;i++)r.push(f(this[i]));return r}
    if (!fn.filter) fn.filter=function(f){var r=[];for(var i=0;i<this.length;i++)if(f(this[i]))r.push(this[i]);return r}
})(Array.prototype);

Put anywhere before first .map or .filter call. Problem solved. jQuery.map() method doesn't work as expected.

UPDATE: I've just tested it on sparse arrays: if map or filter argument is a function which accepts and handles undefined value - it works, but the results are not obvious:

Let's define test sparse array:

var t = []
t[1] = 1; t[3] = 3; t[5] = 5;

Let's see what does IE8 say about t: "[undefined, 1, undefined, 3, undefined, 5]"

Let's try:

t.filter(function(x){return x<4})

What is it, IE8? It's: "[1, 3]". Note - no undefined values. I would personally expect that.

But try THIS:

t.map(function(x){return 2<<x})

And... "[2, 4, 2, 16, 2, 64]". That's weird! :) Try this:

t.map(function(x){return Math.pow(2,x)})

And?... "[NaN, 2, NaN, 8, NaN, 32]" - I would rather expect this result for the previous test. It's at least logical - Math.pow() is supposed to return a number type, NaN, regardless of it's meaning IS a special number type reserved for invalid operations. So the result is more or less correct. It would be fully correct as map result if t remained a sparse array.

So without further ado - ultimately correct version of map and filter methods:

(function(fn){
    if (!fn.map) fn.map=function(f){var r=[];for(var i=0;i<this.length;i++)if(this[i]!==undefined)r[i]=f(this[i]);return r}
    if (!fn.filter) fn.filter=function(f){var r=[];for(var i=0;i<this.length;i++)if(this[i]!==undefined&&f(this[i]))r[i]=this[i];return r}
})(Array.prototype);

And the test:

var t = []; t[1] = 1; t[3] = 3; t[5] = 5;
var t1 = t.map(function(x){return 2<<x});
var t2 = t.filter(function(x){return x<10});
console.debug(t);
console.debug(t1);
console.debug(t2);

Expected results:

[object Array] [undefined, 1, undefined, 3, undefined, 5]

[object Array][undefined, 4, undefined, 16, undefined, 64]

[object Array][undefined, 1, undefined, 3, undefined, 5]

回答4:

MDN says that IE 9 supports it. No mention of IE 8.

回答5:

On MSDN it is said in Requirements for map: Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

Map is just a implementation of "Visitor" pattern for array. So easy substitute for it could be:

function visitArray(arr, visitor) {
    var result = [];

    for (var i = 0; i < arr.length; i ++) {
        result[i] = visitor(arr[i]);
    }

    return result;
}

The function also takes array and function which to invoke on each array element. It returns a new array with result of visitor invokation for each original array element