Mastering JavaScript Functional Programming

Adding missing functions

This idea of defining a function on the run, also allows us to write polyfills that provide otherwise missing functions. For example, let's say that instead of writing code like:

if (currentName.indexOf("Mr.") !== -1) {    // it's a man    ...}

You'd very much prefer using the newer, clearer way, and just write:

if (currentName.includes("Mr.")) {    // it's a man    ...}

What happens if your browser doesn't provide .includes()? Once again, we can define the appropriate function on the run, but only if needed. If .includes() is available, you need do nothing, but if it is missing, you then define a polyfill that will provide the very same workings.

You can find polyfills for many modern JS features at Mozilla's developer site. ...