Using prototype apply vs this to call function

javascript, prototype, this

Solution

The answer is that the version of the polyfill using `this.indexOf` would not conform to the spec for `String.prototype.includes`, which allows `this` to be anything convertible to a string:

If searchString appears as a substring of the result of converting this object to a String...

For instance, the `this` to `includes` could be a number:

<< String.prototype.includes.call(1, '1')
>> true

This is analogous to `String.prototype.indexOf`, which according to spec also does not require its `this` to be a string.

<< String.prototype.indexOf.call(1, '1')
>> 0

If `includes` is implemented as the OP suggests with `this.indexOf`:

String.prototype.includes = function(searchString, position) {'use strict';
    return this.indexOf(searchString, position) !== -1;
};

Then calling `includes` with a non-string `this`, as allowed by the spec, generates a run-time error:

<< String.prototype.includes.call(1, '1')
>> TypeError: undefined is not a function

Whereas the MDN polyfill:

String.prototype.includes = function() {'use strict';
    return String.prototype.indexOf.apply(this, arguments) !== -1;
};

works correctly, taking advantage of the fact that the `this` for `String.prototype.indexOf` also need not be a string:

<< String.prototype.includes.call(1, '1')
>> true

So I imagine the MDN polyfill is written that way not to protect against the `indexOf` method being overwritten on some particular string object, or as a shorthand to avoid having to list out parameters, or due to some Crockfordian preference for the `prototype.apply` idiom, but rather in order to correctly implement the spec.

Problem

Original source