How Does Array.prototype.slice.call Transform Arguments into Arrays?

Unveiling the Mechanism of Array.prototype.slice.call

Array.prototype.slice.call is a versatile tool that grants us the ability to transform arguments into genuine arrays. However, the underlying mechanism behind this transformation may seem enigmatic.

Under normal circumstances, when invoking .slice(), the function expects its input to be an array. It proceeds by iterating over this array, meticulously executing its operations.

What makes this process intriguing is the uncanny behavior of .slice() when applied to arguments. Typically, arguments is not an array itself. Rather, it possesses an array-like structure, boasting a .length property and an assortment of numeric indices. This endows .slice() with the flexibility to assume that arguments is an array, enabling it to carry out its operations seamlessly.

The key to this transformation lies in the call() and apply() methods. These methods empower us to manually assign the this value of a function to a specific object. By setting the this value to our array-like object, we essentially trick .slice() into believing that it is working with an actual array.

To illustrate this concept, consider the following plain object:

var my_object = {
    '0': 'zero',
    '1': 'one',
    '2': 'two',
    '3': 'three',
    '4': 'four',
    length: 5
};

Although my_object is not an array, we can harness .slice() to extract its values by setting it as the this value:

var sliced = Array.prototype.slice.call( my_object, 3 );

As evident from the console output, the resulting array contains the desired elements:

['three','four'];

In essence, when we employ Array.prototype.slice.call with the arguments object, the mechanics are identical. The arguments object, despite not being an array, mirrors its structure sufficiently to deceive .slice() into performing its operations as if it were an array.

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