Question

I’m working my way through the Eloquent JavaScript Book and in it there is the following code:

function createFunction(){
  var local = 100;
  return function(){return local;};
}

When I run this via the node console (run node from command prompt) by calling createFunction(), I get [Function] as a returned value. However, according to the book I should get 100.

So my two questions: Why is this? and Second, is running these little examples in the node console a bad idea for testing JS code?

Answer 1

You need to call the response of createFunction().

createFunction()();

The first invocation (()) calls createFunction() and returns the inner function, which the second invocation executes and returns the local variable which was closed over.

Running small examples in a node console (or any other) is fine, so long as you know the environment, e.g. a browser’s console is generally eval()‘d, which can create side effects, such as how delete can apparently delete variables, not just object properties.