What are free variables?

Question

Javascript closure definition says :

A \"closure\" is an expression (typically a function) that can have free variables together with an environment

Answer 1

A "free-translation" could be: "out of scope" - variables.

Since ECMAscript uses lexical scoping, a free variable is a variable which was defined in a parent-scope and gets looked-up by a scope-chain search.

(function _outerScope() {
    var foo = 42;

    (function _innerScope() {
        var bar = 100;

        console.log( foo + bar ); // 142
    }());
}());

In the above example, foo is a free variable within the context of _innerScope. it becomes very obvious if we have a quick glance into the underlying concepts of ECMAscript.

A Context is linked to an Activation Object (in ES3), respectively a Lexical Enviroment Record (in ES5), which contains things like: function declarations, variables declared with var and formal paramters, as well as a reference to all parent Activation Objects / Lexical Environments. If a variable needs to be accessed, the ECMAscript engine will first look into the AOs / LEs from the current Context itself; if it can't be found there, it looks into the parent AO's / LE's.

Since any Context stores this data in an array-like structure (don't forget we're talking about implementation level here, not Javascript itself), we are talking about Lexical Scope, because we search through all parent Contexts in order.