Closures

the first example code:

function outer() {
    let a = 10
    function fn() {
        console.log(a)
    }
    // console.log(fn) // fn is a function
    return fn
}
// console.log(outer()) 
// outer() is exactly equal to the function fn
// outer() == fn == function fn(){...}
const fun = outer()
fun() // the console prints 10
// console.log(fun()) // the console prints 'undefined', because the function fn doesn't return a significant value  

remember that const fun=outer() not const fun=outer,  in other words, fun is the return value of the function outer

 

in the second example, the inner anonymous function is used as the return value of the function outer:

function outer(){
    let a=10
    return function(){
        console.log(a)
    }
}
// outer() is equal to the anonymous function
const fun=outer()
fun() // call the anonymous function, the console prints 10