Why is Array.prototype.slice.call(arguments) used? How do call and apply work?

Array.prototype.slice.call(arguments) is used to convert the array-like object arguments into a real array. The arguments object is not a full-fledged array, so it doesn't have array methods like slice.

The slice method is usually called on an array, but with call, it can be invoked on any object that has numeric indices and a length property.

call and apply are methods of functions that allow you to invoke a function with an explicitly specified context (this) and arguments:

• call(thisArg, arg1, arg2, ...) — calls the function with this = thisArg and passes arguments separately.
• apply(thisArg, [arg1, arg2, ...]) — calls the function with this = thisArg and passes arguments as an array.

Example:

function sum(a, b) {
  return a + b;
}

sum.call(null, 1, 2); // 3
sum.apply(null, [1, 2]); // 3

Thus, Array.prototype.slice.call(arguments) converts arguments into an array so that array methods can be used.