通过功能和闭包简化创建模块

Question

This is from an example from a popular JS book that is exemplifying creating a module with the use of functions and closures. Simplified, the example looks like this

String.prototype.startify = function ( ) {
 var startString = "Start ";
  return function () { return startString + this;};
}();

console.log("Text".startify());

My question is, can this be reduced to the following since it seems to achieve the same result? Also what is the purpose of returning a function and doing the last () ?

String.prototype.startify = function ( ) {
 var startString = "Start ";
 return startString + this;
};

console.log("Text".startify());

Answer 1

The last () is invoking the function. Imagine you have a function foo

function foo() { }

If you want to run this function you call

foo();

Now if foo returns a function

function foo() {
  return function bar() {
  };
}

You can do

var bar = foo();
bar(); // bar is a function

In you code

String.prototype.startify = function ( ) { // foo
  var startString = "Start ";
  return function () {                     // bar
    return startString + this;
  };
}();      // () executes foo immediately

You are running the function "foo" and returning the function "bar" then storing it on the variable String.prototype.startify .

foo is only called once here, different from your second example.

So as @Xufox said

Imagine var startString = "Start "; being a very complex expression. In your second snippet, that expression would need to run every time you use startify. In the first example, it only needs to run once.

Also another advantage of this is that the variable startString will not pollute the rest of the file or global with a variable you only use in a specific place, this variable will only exist in the function "foo".