what is it used for _.once in underscore?

Question

I just look at the once API of source in underscore.js, then wandering what is it the used for in the method, it seems doing nothing:

func = null

the source:

  _.once = function(func) {
    var ran = false, memo;
    return function() {
      if (ran) return memo;
      ran = true;
      memo = func.apply(this, arguments);
      func = null;
      return memo;
    };
  };

Answer 1

What the function does can be found in the documentation:

Creates a version of the function that can only be called one time. Repeated calls to the modified function will have no effect, returning the value from the original call. Useful for initialization functions, instead of having to set a boolean flag and then check it later.

Why set func = null is explained in this commit message:

Assuming we'll never run the wrapped function again on _.once(), we can assign null to the func variable, so function (and all its inherited scopes) may be collected by GC if needed.

Answer 2

From the official underscorejs website:

once _.once(function)

Creates a version of the function that can only be called one time. Repeated calls to the modified function will have no effect, returning the value from the original call. Useful for initialization functions, instead of having to set a boolean flag and then check it later.

var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.

http://underscorejs.org/#once