Cover by Douglas Crockford

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Functional

One weakness of the inheritance patterns we have seen so far is that we get no privacy. All properties of an object are visible. We get no private variables and no private methods. Sometimes that doesn't matter, but sometimes it matters a lot. In frustration, some uninformed programmers have adopted a pattern of pretend privacy. If they have a property that they wish to make private, they give it an odd-looking name, with the hope that other users of the code will pretend that they cannot see the odd looking members. Fortunately, we have a much better alternative in an application of the module pattern.

We start by making a function that will produce objects. We will give it a name that starts with a lowercase letter because it will not require the use of the new prefix. The function contains four steps:

  1. It creates a new object. There are lots of ways to make an object. It can make an object literal, or it can call a constructor function with the new prefix, or it can use the Object.create method to make a new instance from an existing object, or it can call any function that returns an object.

  2. It optionally defines private instance variables and methods. These are just ordinary vars of the function.

  3. It augments that new object with methods. Those methods will have privileged access to the parameters and the vars defined in the second step.

  4. It returns that new object.

Here is a pseudocode template for a functional constructor (boldface text added for emphasis):

var constructor ...

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