Closure and this - How are variables resolved within a function?

Let's start with a example of closure -

function outer() {
  // scope A
  var myText = 'Hello'
  return function inner() {
    // scope B
    console.log(myText)
  }
}

{
  // scope C
  let myText = 'World'
  let fn = outer()
  fn();
}

Here we have a outer function, which returns a inner function, and this inner function is called later inside another block scope. The scope for each of them is labelled from A to C, and there is also a outermost global scope.

What is the output above? How is the variable myText resolved within the function?

In the same note, how is this resolved within the function body?

Creating and calling function is different

Let's understand in depth, from the start.

A function has 2 distinct phases - function creation and function call.
Function is created when you define it (with hoisting) in the lexical scope or creation scope. Now, this function can be stored in some variable and called much later - from the caller scope. These creation and caller scopes might be different.

All of these sound obvious, but it's easy to forget the difference. So, let's actually see the 2 phases for our inner function -

function outer() {
  // scope A
  var myText = 'Hello'

  // ⭐️ Create phase for `inner`
  // Creation scope = scope A
  return function inner() {
    console.log(myText)
  }
}

{
  // scope C
  let myText = 'World'
  let fn = outer()

  // ⭐️ Call phase
  // Caller scope = scope C
  fn();
}

Here, function inner will get created when outer is called, and the creation scope is scope A. But it is called later in scope C.

When function is created

When a function F is being created, it stores a few internal properties. Two of them are important for this understanding.

  • It stores the current scope (creation scope) in F.[[Environment]]. This is useful for implementing closure behaviour.

  • [[ThisMode]] - For normal functions, it is either global (default) or strict (in strict mode). For arrow functions, it is lexical. For our discussion, what matters is whether it is lexical or non-lexical (the other two).

In this article, we'll only look at how this is resolved - not the actual value of this. For that, please read my previous post.

When function is called

Scope creation

When a function is called, it creates a new execution context. Execution context has a property called LexicalEnvironment (LE). This LE points to the current scope which should be used for lookup. We know that a function has its own new scope. So, a new FunctionEnvironment (function scope) is created and set to LE.
✅ So, till now - when a function is called, it gets a new execution context, whose LE points to a new function scope. This new scope will contain function's own local variables.

✅ Now, scopes are chained - so, this new function scope (LE) needs to decide what is it's parent scope (outerEnv). It has 2 options -

  1. caller scope (which was the current scope before this new one was created) or,
  2. it's lexical scope (which it is carrying around in F.[[Environment]] ).

It decides to set the lexical scope as parent, ignoring the caller scope.

Looking back at our code, that means -

function outer() {
  // scopeA
  var myText = 'Hello'
  // inner.[[Environment]].[[LE]] = scopeA
  return function inner() {
    // new scope = scopeB
    // ⭐️ scopeB -> scopeA -> global
    // ❌ Does NOT have scopeC in chain
    console.log(myText)
  }
}

{
  // scope C
  let myText = 'World'
  let fn = outer()
  fn();
}

Variable lookup

Now, when we use something like console.log(myText) in the function body - it needs to resolve or lookup the variable myText.

For variable lookup,
a. it will check in the current LexicalEnvironment (i.e. the new local scope) first, and
b. if it doesn't find the variable there, it will check in it's parent scope (parent of LE = F.[[Environment]] = lexical/closure scope),
c. and so on.

This lookup will finally end when it reaches the global scope (which doesn't have any parent scope). If the variable is still not found, it will throw a ReferenceError.

If you remember the scope chain of inner func, this means that the variable will be looked up in lexical/creation scope (scopeA) and NEVER in caller scope (scopeC). This behavior is called as closure or lexical scoping.
Even if there is no definition for myText in the lexical scope (scopeA), it will NOT use the value from scopeC - and indeed throw a error. Example ⤵️

function outer() {
  // scopeA
  return function inner() {
    // scopeB
    // ⭐️ throws ReferenceError
    console.log(myText)
  }
}

{
  // scopeC
  let myText = 'World' // always ignored
  let fn = outer()
  fn();
}

(Actual implementation of closure in different engines will optimize the closure by only storing variables that are actually being used (if any). But the Ecmascript specification doesn't talk about those optimizations and tells you to directly store the whole lexical scope, for all functions.)

this lookup

Earlier, we looked at how normal variables get resolved in a function. But whenever you write this.someThing, the value of this does NOT get resolved in the same way. It is known as a ThisExpression, which gets resolved specially.

Before going further, lets take a small detour.

Remember the function scope (FunctionEnvironment) and F.[[ThisMode]]? Well, this FunctionEnvironment also has 2 internal properties - env.[[ThisBindingStatus]] and env.[[ThisValue]]. Their values are set based on F.[[ThisMode]].

If [[ThisMode]] is lexical, set [[ThisBindingStatus]] to 'lexical' and no need of setting [[ThisValue]].
If [[ThisMode]] is non-lexical, set [[ThisBindingStatus]] to 'initialized' and set [[ThisValue]] to the actual this that the function was called with.

In summary, the function scope has 2 properties which say - whether the scope has a this value or not and if it has, then what is the value. Arrow functions will not have any this value, but normal functions will.

This FunctionEnvironment also has 2 helper methods -

  1. HasThisBinding() - Returns false if [[ThisBindingStatus]] is 'lexical'. Else, returns true.

  2. GetThisBinding() - Returns env.[[ThisValue]]. This will only be called if HasThisBinding() is true.

These two methods are implemented on all types of environments (scope). Here is a summary of what these methods return on other environments -

Environment TypeHasThisBinding()GetThisBinding()
functiontrue/false[[ThisValue]]
block scopefalse-
moduletrueundefined
globaltrueglobal object
objectfalse-

Resolving logic

Now, back to the special resolving logic for this. It will do -

Starting from the FunctionEnvironment (i.e. current function scope),
a. if env.HasThisBinding(), then return env.GetThisBinding()
b. else, move up to parent scope and repeat.

It will finally reach global scope, which always provides a this value (global object). For modules, it will stop at the module scope - whose this value is undefined.

In short, it means that the value of this will be resolved from the closest this-providing scope. (Follow the table above.)

What does this explain?

A. Arrow functions don't have their own this value. It is taken from the closest this-providing scope.

function outer() {
  const inner = () => {
    // prints `obj`
    // which is this value in outer function
    console.log(this)
  }
  inner();
}

const obj = {abc: 123};
outer.call(obj);

Same logic also applies for nested arrow functions.

B. Block scopes also don't have their own this value.

{
  // just prints global object
  console.log(this)
}

C. In modules, top-level this is undefined.

// Save as file.mjs and run in node.
// That will run it as a module.

console.log(this); // prints `undefined`

That's all. Thanks for reading. I post on LinkedIn regularly about more javascript stuff.

Bolaji Ayodeji's photo

This is really amazing, thanks for sharing!

Lakshya Thakur's photo

Explanation - 💯. Much insightful !