JavaScript is fortunate to number itself among the programming languages that support inner function declarations. Many traditional programming languages, such as C, collect all functions in a single top-level scope. Languages with inner functions, on the other hand, allow us to gather small utility functions where they are needed, avoiding namespace pollution.

An inner function is simply a function that is defined inside of another function. For example:

function outerFun() {
  function innerFun() {
    alert('hello');
  }
}

The innerFun() is an inner function, contained within the scope of outerFun(). This means that a call to innerFun() is valid within outerFun(), but not outside of it. The following code results in a JavaScript error:

function outerFun() {
  function innerFun() {
    alert('hello');
  }
}
innerFun();

We can trigger the alert, though, by calling innerFun() from within outerFun():

function outerFun() {
  function innerFun() {
    alert('hello');
  }
  innerFun();
}
outerFun();

This technique is especially handy for small, single-purpose functions. For example, algorithms that are recursive but have a non-recursive API wrapper are often best expressed with an inner function as a helper.