TypedArrayConstructor.js

function from(items /* [ , mapfn [ , thisArg ] ] */)
{
    "use strict";

    if (!@isConstructor(this))
        @throwTypeError("TypedArray.from requires |this| to be a constructor");

    var mapFn = @argument(1);
    var thisArg;
    if (mapFn !== @undefined) {
        if (!@isCallable(mapFn))
            @throwTypeError("TypedArray.from requires that the second argument, when provided, be a function");

        thisArg = @argument(2);
    }

    var arrayLike = @toObject(items, "TypedArray.from requires an array-like object - not null or undefined");

    var iteratorMethod = items.@@iterator;

    if (!mapFn && @isTypedArrayView(arrayLike)) {
        var arrayLikeLength = @toLength(arrayLike.length);

        var result = new this(arrayLikeLength);
        if (@typedArrayLength(result) < arrayLikeLength)
            @throwTypeError("TypedArray.from constructed typed array of insufficient length");

        // This is not precise enough I think?
        this.prototype.set.@call(result, arrayLike);
        return result;
    }

    if (!@isUndefinedOrNull(iteratorMethod)) {
        if (!@isCallable(iteratorMethod))
            @throwTypeError("TypedArray.from requires that the property of the first argument, items[Symbol.iterator], when exists, be a function");

        var accumulator = [];

        var count = 0;
        var iterator = iteratorMethod.@call(items);

        // Since for-of loop once more looks up the @@iterator property of a given iterable,
        // it could be observable if the user defines a getter for @@iterator.
        // To avoid this situation, we define a wrapper object that @@iterator just returns a given iterator.
        var wrapper = {};
        wrapper.@@iterator = function() { return iterator; }

        for (var value of wrapper) {
            @putByValDirect(accumulator, count, value);
            count++;
        }

        var result = new this(count);
        if (@typedArrayLength(result) < count)
            @throwTypeError("TypedArray.from constructed typed array of insufficient length");

        for (var k = 0; k < count; k++) {
            var value = accumulator[k];
            if (mapFn)
                result[k] = thisArg === @undefined ? mapFn(value, k) : mapFn.@call(thisArg, value, k);
            else
                result[k] = value;
        }

        return result;
    }

    var arrayLikeLength = @toLength(arrayLike.length);

    var result = new this(arrayLikeLength);
    if (@typedArrayLength(result) < arrayLikeLength)
        @throwTypeError("TypedArray.from constructed typed array of insufficient length");

    for (var k = 0; k < arrayLikeLength; k++) {
        var value = arrayLike[k];
        if (mapFn)
            result[k] = thisArg === @undefined ? mapFn(value, k) : mapFn.@call(thisArg, value, k);
        else
            result[k] = value;
    }

    return result;
}