Closure in JavaScript – Part 2: Definition and usage

Tram Ho

You can read through part 1 here

In order for everyone not to forget, I briefly summarize the concept of the lexical environment:

Lexical Environment is an anonymous object found in every object in JavaScript, it contains variables in a scope and references to the external environment.

Oke? Now comes the definition of Closure:

A Closure is a function that can remember and access its lexical environment even outside of that lexical environment .

Let’s see the following example:

We can see that the stuff function is returned by the say function, and we assign s to the stuff function itself (not the return value of the stuff function).

So, the stuff function, after being assigned to s , is no longer running in the lexical environment it was declared with.

After the say function is run, we would think the information in its lexical environment will disappear, but because the stuff function still exists and contains a connection to the external lexical environment (here the say function, This information is still kept in. Someone calls this connection chain the closure .

Closures in JavaScript are more common than you might think

We will take the following 1 example:

Here is an example taken directly from the book You Dont Know Javascript . Anyone who has ever touched Javascript has used the setTimeout function. In this example, we have an innermost function called timer and will pass it into setTimeout to run after 1 second. This is a usage that I think is quite popular. Just like in the previous example, you might think that after 1 second, the message parameter of the wait function should have disappeared, but the timer function here still has a string connected to the lexical environment of wait , and prevents message is deleted by the Garbage Collector.

Closure and loop

What do you think the following loop will print out?

The real result is that it will print the number 6 5 times, every 1 second. Ie the i that the setTimeout receives is different from the i that the timer receives, why is that?

First, where did 6 come from? It results when the loop encounters a condition that does not satisfy i <= 5 , ie i = 6 , then it terminates and the new setTimeout function is run. And if you replace i*1000 with 0 , the setTimeout function will still run after the loop completes. So how to print exactly as I want?

What we want here is probably that each loop captures its own variable i , but for now, all 5 iterations of this loop are sharing the same external lexical environment that has only one i when the loop is running. accomplished. So what we need is a lexical environment that can contain this variable i .


This syntax:

As an IIFE , using an IIFE in a loop creates a new lexical environment around the setTimeout function in each round, giving it the variable i it needs.

What is the closure used?

There are many real life situations that often use closures, you should try to read the Javascript I wrote to see if there is any place to use it. Many people use it to write thedebounce and throttle functions

Those are the most basic things that I know about Closure. You can refer to here and here . One more interesting thing with the example above

When you replace var with let , another behavior happens. You try to find out for yourself why that is okay.

Thanks for reading.

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Source : Viblo