As Easy As Closure

Aanchal Kapoor
May 23, 2021 · 13 min read
Credit: frank mckenna in unsplash

Closure has been baffling mankind since its inception in JavaScript😉😁And yet if you look at it closely to understand how it works, it proves to be an incredibly elegant and simple construct. In this blog, along with theory, I am going to walk you through a handful of examples to build that mental, conceptual model that (hopefully) sticks around in your head, and you will never wonder what closure are, again.

These are the concepts that we are going to get comfortable with in this blog:

  • Thread of execution
  • Execution context
  • Call stack
  • Higher order functions
  • Lexical scope and how JS is a lexically scoped language
  • Closure

Let us start with a basic example first to understand how JavaScript’s thread of execution works:

  • When this program executes, first the function definition for createFunctionfrom lines 1 to 6 is stored in the global memory, with the identifier createFunction.
  • Then in line 8, the identifier generatedFunction is created, and is uninitialised until the createFunction runs and returns. We know that createFunction is about to run and not just referenced, because of the paranthesis with it in line 8.
  • When the createFunction is going to run, a new execution context will be created for it in memory, and createFunction() will be added to the call stack. This is how the call stack looks at this moment:
The current state of the call stack
  • Note that the global() is always at the bottom of the call stack.
  • Then the definition of createFunctionwill be looked up in the (global) memory and execution will start. Note that the thread of execution will not go from line 8 to 1. The thread just keeps moving ahead. To execute createFunction its definition is fetched from the memory.
  • Inside createFunction, first the definition of multiplyBy2is stored in the local memory of createFunction(you can have a look at lines 2 to 4 to make a mental model of this, but the thread of execution, as I said before does not actually go to line 2–4).
  • Then the definition of multiplyBy2is picked up and returned to the calling environment, and stored in the generatedFunctionvariable. In effect that means that the definition of multiplyBy2gets a new label: generatedFunction
  • Once the returnkeyword is hit on line 5 (that again means that you can look at line 5 to read what is returned but the thread of execution does not actually go to line 5 — it simply returns the multiplyBy2 function when it encounters the return keyword in the function definition of createFunction coming from memory), the execution context of createFunctionis deleted, and so is its entry on the call stack.
The current state of the call stack
  • Next in line 10, the identifier resultis created and is unitialised till the generatedFunction runs and returns.
  • A noteworthy point here is that the original environment where the definition of generatedFunctionwas created no longer exists. It is not even needed because the definition of multiplyBy2 now lives in the global memory under the label generatedFunction .
  • generatedFunctionis run, its own execution context is created, and its entry added to the call stack.
Current state of the call stack
  • Inside the execution of generatedFunction, first the argument 5 is stored in its local memory with the label number.
  • Then the computation of number * 2 is done which evaluates to 10. The value 10 is then returned into the calling environment, and stored in the variable result.
  • In the meanwhile, the execution context and the call stack entry for generatedFunction are deleted.
The current state of the call stack
  • In line 11, the value of result is logged out.

A noteworthy point in this discussion is that:

In line 5, the definition of multiplyBy2 and not its reference, was returned. Had the reference been returned, how would its invocation (with a new label generatedFunction) in line 10 be possible? Remember, the execution context and local memory of createFunction were deleted as soon as the function returned!!

So now we know what execution context, call stack are, and how functions are returned from other functions.

By the way, functions that accept and/or return other functions are called higher order functions.

Let us move ahead with another example:

This example builds on top of the basicExample.js. For the sake of completeness, let us go through the mechanism of the thread of execution once again, a bit more quickly this time, because we understood it in a great detail the last time.

  • Lines 1–10: definition of createFunctionstored in memory.
  • Line 12: identifier result created, is still uninitialised till createFunction runs and returns. createFunction starts to run, together with the creation of its own execution context and its entry in the call stack.
The current state of call stack
  • Inside the execution of createFunction , identifier value is created with the value 5. The definition of multiplyBy2 is stored with the identifier multiplyBy2. All this is stored in the local memory alloted to createFunction.

Note again: when the function createFunction is run, the thread of execution NEVER goes back from line 12 to line 1. When it has to execute this createFunction , it executes it from the definition of createFunction stored in the memory. I keep referring back to line 1 from 12, because this is how we see what is happening.

  • Further when multiplyBy2 is invoked, a new execution context is created for it and its entry pushed to the call stack.
The current state of the call stack
  • Inside the execution context of multiplyBy2 , the value of value has to be updated to its double. But value does not exist in the local memory/execution context of multiplyBy2 .

What does the JS engine do in this situation?

Does it move down the call stack to look for the definition of value?

You might be tempted to think yes, it does!

Alright, to make you happy I will assume for a minute that it does. So assumably, the definition of value is found in the local memory of the parent context (that of createFunction) and updated to be double of itself.

  • Next, the closing brace of multiplyBy2 is hit, the function ends, its execution context is deleted and it is popped off the call stack.
The current state of call stack
  • Further in the execution of createFunction, the value of value is returned to the calling environment.
  • Line 12: Now the return value from createFunction()is stored with the identifier result in the global context. In the meanwhile, the execution context of createFunction too is deleted and it too is popped off the call stack.
The current state of call stack
  • Line 13: The value of result is printed to the console.

Now coming back to the point where we made a big assumption: when a function cannot find the definition of a variable in its own local memory it keeps going one step at a time down the call stack to stop at the first location where it finds that variable definition.

Did we do the right thing in assuming that?

Instead of building up more suspense, I will simply delve into another example which can clarify things up.

I would like to leave the discussion of how the thread of execution proceeds and how the execution context and the call stack are handled as an exercise for you. You can refer to the discussions above to do it 😊

The point to be focused on in here is this:

In line 11, the createFunction function returns the definition of multiplyBy2 function and stores it with the new label doubleItUp. This returned function has references to a variable (value) that is not defined inside its own local memory (see lines 4 to 7, but the thread of execution does not actually go there), but defined in the local memory of the context where multiplyBy2 was defined (i.e., in the context of createFunction , i.e. its parent context).

Further, when this returned function is actually invoked, the context of createFunction does not even exist any longer!!!

So how do we expect this returned function which is now stored in the global memory with the label doubleItUp to run successfully and process the variable value ?

So what happens at line 13-14? If our earlier assumption were true, then when doubleItUp is invoked, it tries to double the value of value and return it. But in the first place, it cannot find the variable in its own execution context, so it (according to our assumption) tries to look one level down the call stack to find the variable. But at one level down the call stack, at this stage is… drum roll… the global context!! Because the call stack entry of createFunctionhas long been deleted when createFunction returned!!

Digest this point for a moment. If needed go through the running of the execution context once more for this example…

Ready for the next thing? Alright, now that we are sure that the value variable cannot come from the parent scope of multiplyBy2 and that this example does run successfully, the question arises: from where does the definition of value come??

The answer, which you probably might have guessed is… from the CLOSURE of multiplyBy2. Aha! Finally we got there!!

But wait a second. We do not know what a closure is in the first place.

So let us talk about it more formally.

When multiplyBy2 or for that matter any function, is created, it gets a small store of memory that holds the variables to which that function refers to in its definition. In JavaScript, this store of memory and also the overall concept is called Closure. It is like the surrounding environment is put inside a box, closed over and shipped together with the function, wherever the function goes.

The values held by the closure of a function come from the lexical scope of the function. The lexical scope of a function is the environment in which the function was created.

Let us correlate this with our example, closuresInFullForm.js.

When createFunction returned multiplyBy2, it did not only return the definition of multiplyBy2 , but also a small store of references it held to in its definition. So this store contained the value field and was shipped back into the global context along with the definition of multiplyBy2.

After that as expected the definition of multiplyBy2 got a new label: doubleItUp but the store (aka the closure) remained intact and unchanged. Whenever we wanted to run doubleItUp which was formerly multiplyBy2, the JS engine would need the reference to value, which it first looked for in:

  • the (newly created) execution context of doubleItUp , did not find it there.
  • then looked up in the closure found it there and processed it. Hence the output in line 14 is 10.

Phew!! That was quite some discussion.

To visualise how closure is created, you can think of the closure (of say processValues function) as being an octopus with its tentacles sticking out to the references in the environment where processValueswas created. This environment is the lexical scope of processValues.

Closure is analogous to an octopus’ tentacles
Output

Important points to note in the last example:

  • The fields in the local environment of processValuesfunction which were not referenced by processValues were not added to the closure. This is an optimisation measure adopted by the designers of JavaScript. So value4 is lost forever once createFunction finishes execution.
  • The data stored in the closure store is private and cannot be accessed directly in any way. Only when doCalculations runs, do the values of value1 , value2, and value3 get updated. See output above.
  • It is the hidden [[scope]] property of the returned processValues function that makes the mechanism of closure possible.
  • This is the order in which the JS engine goes looking for references:
The order in which the JS engine goes looking for references

This is because each function that is created in JS, holds its closure and this closure originates from the place where this function was originally defined, that is, the lexical scope of the function. So functions have, in a way some rememberance of their place of birth. This lexical scope has nothing to with the environment/scope where this function was called / invoked.

Here is an example:

  • When in line 19 createFunction runs, it returns the processValues function definition plus its closure.
  • When doCalculations runs in line 21, its closure only remembers the lexical scope where it was defined originally. So its closure contains value1, value2 and value3 from the (now deleted) execution context of createFunction . Remember the execution context of createFunction was deleted but the values value1, value2 and value3 persisted in the closure.
  • doCalculations was called/invoked in the execution context of global. The data in global has no impact on the running of doCalculations . So value3 in the global scope has no effect on doCalculations.

Just a side note, had JS been a dynamically scoped language, the scope of the environment where the function was invoked would have had an impact on the running of a function.

Flexing our closure understanding-muscles with some more examples

Updating closure variable multiple times

Output:

Line 13 prints 1.

Line 14 prints 2.

Need a quick explaination?

Line 11 runs outer and stores the function definition and closure of inner with the label newFunction. When newFunction is run in line 13, it starts looking for the definition of counter and finds it in the closure, counter’s current value is 0, updates it to 1 and prints it. When newFunction is run again in line 14, it again finds the definition of counter in the closure, this time counter’s existing value is 1, which is updated it to 2 and printed.

This is a handy way of remembering data from the previous runs of functions, formally known as memoization. Had newFunction involved a complex and time consuming calculation, it would have been easy for the JS engine to remember the previous value from the previous run and to continue from that point in the next invocation.

Output:

Pretty simple stuff, I leave the explaination of this example as an exercise to you.

The only special thing to note is that each function (inner1, inner2 and inner3)in the array of returned functions has access to the exact same closure. So multiple functions returned together from a higher order function have the same closure.

Now things are getting more exciting. In line 11, the inner function was returned with its closure and given a new label newFunction1. Invocations of newFunction1 on lines 12 and 13 update the counter variable in their closure and print it as 1 (in line 12) and 2 (in line 13).

Line 15 contains a fresh invocatoin of outer which returns a fresh copy of inner and and a fresh instance of closure, which gets a new label newFunction2. So lines 16 and 17 print 1 and 2 again.

The closures associated with newFunction1 and newFunction2 are isolated from each other.

This time the output is: 1 on each of lines 12, 13, 16 and 17.

Line 11: outer returns and the definition and closure of inner function are stored with the label newFunction1. Each time newFunction1 is invoked in lines 12 and 13, the function runs, but this time it need not go hunting for the definition of counter in the closure. Its definition is found right there in the newFunction1’s own local memory. Each time the newFunction1 is invoked, counter is initialised to 0 afresh and incremented to 1.

In line 15, a new copy of inner is returned with a new instance of closure to the label newFunction2. The same process repeats again. Again, the thread of execution does not have to look beyond the local memory of newFunction2 for the definition of counter. Each invocation of newFunction2 in lines 16 and 17 initialises counter to 0 afresh and increments it to 1 each time.

In both the above cases, the concept of closure is redundant.

Output:

Line 13: 1

Line 13: 2

Line 13: 3

Line 13: 4

This happens because this time each invocation of the returned inner function, in the form of newFunction1 and newFunction2 updated and printed the global variable counter.

Why? Because when newFunction1 or newFunction2 are invoked, they can not find the definition of counter in their local memory or closure. So the JS engine looks for counter in levels down the call stack, finds it in global and updates it.

Closure is important to understand several features of JavScript, like the following.

  • Memoization: As explained above, giving our function persistent memory of their previous inputs and outputs.
  • Iterators and Generators: Use lexical scoping and closure to achieve the most contemporary patterns for handling data in JS.
  • Module Pattern: Preserve state for the lifetime of an application without polluting the global namespace.
  • Asynchronous JavaScript: Callbacks and promises rely on closure to persist state in an async environment.

That is all folks. I know that was quite a lengthy read, but hopefully it cleared your mind about this esoteric feature of JS. If this feels quite a lot, I suggest you work through each of the examples with a pen and a paper and see how the thread of execution moves around.

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Aanchal Kapoor

Written by

A full stack programmer with a love for writing and teaching.

Nerd For Tech

NFT is an Educational Media House. Our mission is to bring the invaluable knowledge and experiences of experts from all over the world to the novice. To know more about us, visit https://www.nerdfortech.org/.