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In JavaScript, the special arguments variable holds the arguments passed to the current function. Although this is conceptually an array of items, it is not actually a JavaScript Array type. It can be useful to coerce it to an Array object so you can iterate through it without thinking about its special quirks. The code/**
* Returns a new array from a segment of an array. This is a generic version of
* Array slice. This means that it might work on other objects similar to
* arrays, such as the arguments object.
*
* @param {Array} arr The array from which to copy a segment.
* @param {number} start The index of the first element to copy.
* @param {number} opt_end The index after the last element to copy.
* @return {Array} A new array containing the specified segment of the original
* array.
*/
goog.array.slice = function(arr, start, opt_end) {
// passing 1 arg to slice is not the same as passing 2 where the second is
// null or undefined (in that case the second argument is treated as 0).
// we could use slice on the arguments object and then use apply instead of
// testing the length
if (arguments.length <= 2) {
return Array.prototype.slice.call(arr, start);
} else {
return Array.prototype.slice.call(arr, start, opt_end);
}
};The explanationThe entire point of this function is to get function arguments as a JavaScript Array. We take advantage of the fact that the methods of the Array prototype doesn't actually care whether their first argument is an Array; they will coerce the object for us and return a new Array with its items. After we know that, the only trick is to handle an optional argument in case we want to reuse this function to return only part of an array or array-like object. goog.array.slice = function (arr, start, opt_end) {
if (arguments.length <= 2) {
return Array.prototype.slice.call(arr, start);
} else {
return Array.prototype.slice.call(arr, start, opt_end);
}And here's how you would call it to convert arguments to an Array: var args = goog.array.slice(arguments, 0); Further reading
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The second parameter of Array.prototype.slice is NOT treated as 0 if it's undefined. According to ECMAScript-262 (third edition) section 15.4.4.10, the slice(start, end) method returns an array containing the elements of the array from element start through the end of the array if end is undefined. I see no point in wrapping Array.prototype.slice.
goog.array.slice is not required as Array.slice() is provided by some browsers. YOu could use the above implementation for MSIE.