Closure in JS

The closure is a function that is bound together with its lexical environment.

• Lexical environment is an environment where the function can access a variable declared in its parent environment
• Closure gives access to scope of outer function from inner function.

To understand this, we first need to understand how execution context is created and how call stack works

• Whenever javascript function runs, execution context is created called global execution context and it is put it in call stack.
• Whenever a function is invoked or called , it is place into call stack by creating the function's execution context.
• As we know that stack is LIFO (Last In First Out ) data structure, the data which is put on top of stack comes out first

• Whenever a function is put into stack , it has information about where it is currently present in code, that is its lexical environment.
• Consider below code snippet, the function outerFunc() is called on line 9

Example 1 :

• The OuterFunc() has a function called innerFunc() inside it which it returns, In javascript we can use function name as variable name to return like this.
• InnerFunc() doesn't contain any variable declaration or function definition of its own
• If we see output in screenshot,on line 10 whole innerFunc() is returned.
• if we invoke the return value as function , we can see innerFunc() is getting executed, though the outerFunc() ends on line 7, the value of a is accessible in inner environment through lexical scoping and therfore a's value is printed on line4.

• Can you guess the output of the below snippet ?

Example 2:

• The temptation to answer will be 7, but all things change because of line 6 and makes output as 8 How? lets see
• On line 10, outerFunc() is invoked which gets in return the value of innerFunc() same as example 1
• What differs is on line 6, value of a is reinitialized as 8. Though the innerFunc() can access the value of a as we have seen in example1. It doesn't store a value of a, but has reference to address of a.
• As of now the address of a is the same as the previous but the value is now changed to 100.therfore, output changes.

• Thus , in a way closure provides a superpower for accessing data.