Mastering JavaScript Functional Programming - Second Edition

Wrapping a value – a basic container

Let's pause for a minute and consider what we need from this wrapper. There are two basic requirements:

• We must have a map() method.
• We need a simple way to wrap a value.

To get started, let's create a basic container. Any object containing just a value would do, but we want some additions, so our object won't be that trivial; we'll explain the differences after the code: 

const VALUE = Symbol("Value");class Container {  constructor(x) {    this[VALUE] = x;  }  map(fn) {    return fn(this[VALUE]);  }}

Some basic considerations that we need to keep in mind are as follows:

• We want to be able to store some value in a container, so the constructor takes care of that.
• Using a Symbol helps hide the field: the property ...