The Wacky World of Closures & What Makes Them Useful

Can you guess what this prints out?

for (let i=0; i<3; i++) {
  setTimeout(() => console.log(i), 2000)
}

... Are you sure? There's a gotcha here, and if you're not aware of it already, I'd wager this blog post is worth your time. (Hint: 0, 1, 2 is incorrect.)

Getting Closure With Javascript

To understand what's happening in the above code snippet, we have to understand closures. If you're looking for practical applications of closures, you can jump ahead.

A closure has a surprisingly simple definition: a function with access to information outside of itself, otherwise known as its "lexical environment". function addTwo() is a closure:

let x = 5;
function addTwo() {
    return x+2;
}

And let x = 5 is in its lexical environment.

All functions in Javascript can be closures, because they automatically gain access to outer scope.

By contrast, pure functions are not closures:

function addNums(a,b) {
    return a+b;
}

addNums does not reference any data outside of its own scope. Its data is kept in your computer's short term memory. It gets pushed onto the "call stack", executed, and then popped off the stack again. Clean, simple, easy.

On the other hand, when a function references information outside of its own scope (as with a closure), its data becomes packaged (or "enclosed") with references to all of its lexical info, and the entire package gets placed in longer term memory, called the heap. We can thank a memory management process called garbage collection for keeping the heap clear of information we no longer need in long term memory.

Despite closures needing more memory and computational power, there are some great reasons to use them (which I'll cover in a moment below).

Not All Closures Are Made The Same

Closures are particularly easy in Javascript.

You can use let over lambda to create a closure in Lisp (the second oldest higher-level programming language).

The nonlocal keyword is helpful to gain access to variables normally outside of scope in python closures.

In C# however, closures must explicitly be enclosed with its lexical environment, through "binding" variables.

You get the idea. For now, we'll continue to use Javascript.

What Makes Closures Uniquely Useful?

There is surprisingly sparse info online about uses for closures. It's odd! While I'm sure there are many more uses, there seem to be at least two compelling ones I'd like to discuss:

• Function factories
• Namespacing private functions

Function Factories

Function factories are functions that return other functions based on various conditions. I'd like to share how I used a function factory in a recent project. But first, let's look at a simple example.

function factory(num){
  switch(num){
    case num > 5:
      return (b) => num - b
      break;
    case num = 5:
      return (b) => num % b
      break;
    case num < 5:
      return (b) => num + b
      break;
    default:
      break;
  }
}

If we call factory(5), it returns (b) => 5 % b.
If we call factory(4) it returns (b) => 4 + b.
And if we call factory(4)(2) we can see that:

factory(4) = (b) => 4 + b

So factory(4)(2) becomes ((b) => 4 + b)(2)

Resulting in (2) => 4 + 2. Which returns 6.

The important note here is that function factories return functions that can accept even more info.

A Closure Function Factory In Use

I recently built a notes app with a react front end using semantic-ui-react. The new note form included a dropdown menu. (Bear with me here.)

semantic-ui-react's dropdown menu requires an array of options. Once I fetched data from my database and generated the options array, it looked something like this:

let options = [
    {id: 1, key: 1, text: option1}
    {id: 2, key: 2, text: option2}
    ...
]

You can feed this array to the dropdown like so:

<Dropdown
    name="dropdown"
    multiple
    search
    selection
    options={options}
/>

(I've simplified all of these snippets of code for readability.)

This dropdown will allow you to make multiple selections. It turns out the value attribute of semanitic-ui-react's dropdown menu is an array of ids from the objects in options. I wanted to store whole objects from options in state instead.

I wanted just one change handler function for all form elements. Closure to the rescue.

Every form element executes the same function on change, like this:

onChange={(e) => handleMaker("name")(e)}

"name" matches the name attribute of the form element it's associated with for style reasons.

handleMaker is a function factory that returns a different function based on which form element name is passed in. The function returned from handleMaker accepts the onChange event as an argument.

Here is a simplified version of the function factory I use in the app:

function handleMaker(name){
  switch (name) {
    case "note":
      return (e, {value}) => setFormData({...formData, [name]: value});
    case "collections":
      return (e, {value}) => {
        setFormData({...formData, [name]: value.split(",").map(w=>w.trim())});
      }
    case "dropdown":
      return (e, {value}) => {
        setFormData({...formData, [name]: options.filter(o => {
          for (v in value) {
            return (v === o.id) ? true : false
          }
        })})
      };
    default:
      console.error("Oops, something went wrong!");
      break;
  }
}

There are other cases here, showing how a function factory can help handle all sorts of special cases.

Namespacing private functions

Private functions make apps more secure, disallowing ill-intentioned users from calling functions or methods that can mutate the app's state unhelpfully (or, in some cases, even inject code).

Ruby has a private keyword to make methods private. Javascript didn't until recently. But that applies to classes. When we're not inside classes (or running on IE, lol), we can still namespace private javascript functions with closures:

const namespacer = (function() {
  let num = 100;
  function changer(amt) {
    num += amt;
  }

  return {
    public1: function() {
      changer(100);
    },
    public2: function() {
      changer(-100);
    },
    public3: function() {
      return num;
    }
  }; 
})()

Here, we can see that namespacer is actually an object with closures as keys, since the anonymous function on line 1 is immediately invoked on the last line.

We can call the public functions like this:

namespacer.public1(); // 200
namespacer.public2(); // 100
namespacer.public3(); // 100

But we would be unable to call changer directly:

namespacer.changer(); // TypeError: undefined is not a function

Or access num:

namespacer.num; // undefined

Presto! Private functions.

Closures In Interviews

If you are new to web dev and preparing for interviews, it may interest you to know that there is a common interview question involving closures:

for (let i=0; i<3; i++) {
  setTimeout(() => console.log(i), 2000)
}

Can you guess what console.logs here?

If you guessed

2
2
2

... you'd be right! We might expect 0, 1, 2 but that won't happen here. Each time we go through the loop, setTimeout waits a whole 2 seconds before running. The i inside of the setTimeout callback function refers to the i from the loop. In 2 seconds, the loop will have run all 3 times, leaving i at 2 when all three setTimeouts eventually run.

There are a number of ways we can fix this. One way is to wrap the callback function inside of setTimeout in an immediately invoked function that accepts i as its argument:

for (let i=0; i<3; i++) {
  setTimeout(((i) => {
    () => console.log(i)
  })(i), 2000)
}

What does this accomplish? Wrapping the callback function in an immediately invoked function ensures that the current value of i is passed in and kept in the state of the setTimeout function. It is stored there for later use.

Another way we can do this involves wrapping the entirety of the setTimeout in the same immediately invoked function:

for (let i=0; i<3; i++) {
  ((i) => setTimeout(() => console.log(i)), 2000))(i)
}

This accomplishes the same result.

A Final Musing:

I'm curious to know whether there is a language in which creating a closure is impossible. So far my Googling efforts haven't gotten me far. I'd be grateful for your thoughts on the topic.