Array-like Objects Back

In JavaScript, we will always know that all the things are constructed as objects, and so-called array-like objects means the kind of objects which is like an array, of which arguments is an example.

For this kind of objects, we can access items within them through using operators [], and we can also get the length of this objects by accessing the property of length. However, they are always not supported for using operations of an array like Array.prototype.push(). So how does array-like objects defined in underscore?

var MAX_SAFE_INTEGER = Math.pow(2, 53) - 1;
var getLength = property('length');

var isArrayLike = function (collection) {
    var length = getLength(collection);
    return typeof length === 'number' && length >= 0 && length <= MAX_SAFE_INTEGER;
};

The answer is so simple. If an object has the property length, it's assumed as array-like objects. According to https://tc39.github.io/ecma262/#sec-number.max_safe_integer, Math.pow(2, 53) - 1 is equal to Number.MAX_SAFE_INTEGER, which represents for the maximum of a safe integer value.

Note that: the maximum of the length of an array or an array-like object is Math.pow(2, 32) - 1.

As defined, NodeLists, HTML Collections, and Strings all belong to array-like objects.

When it comes to converting them into a real array, the most simple way is to create a new array and push all items into it:

function arrayFrom(collections) {
    var arr = [];

    for (var i = 0, len = collections.length; i < len; i++) {
        arr.push(arguments[i]);    
    }

    return arr;
}

If you don't need to consider the compatibility of older version IE, you can just use Array.prototype.slice() to complete it:

function arrayFrom(collections) {
    var arr = Array.prototype.slice.call(collections);

    return arr;
}

Or we can even use a literal to make it more simple:

function arrayFrom(collections) {
    var arr = [].slice.call(collections);

    return arr;
}

ES6 has also provided an alternative for you:

function arrayFrom(collections) {
    var arr = Array.from(collections);

    return arr;
}