Node.js by Example: Closures

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives a function access to its outer scope. In JavaScript, closures are created every time a function is created, at function creation time.

myFunc is a reference to the instance of the function displayName that is created when makeFunc is run. The instance of displayName maintains a reference to its lexical environment, within which the variable name exists.

 
function makeFunc() {
    const name = "jsbyexample";
    function displayName() {
        console.log(name);
    }
    return displayName;
}
 
const myFunc = makeFunc();
myFunc();

We have defined a function makeAdder(x), that takes a single argumentx, and returns a new function. The function it returns takes a single argumenty, and returns the sum ofxandy`.

 
function makeAdder(x) {
    return function(y) {
        return x + y;
    };
}
 
const add5 = makeAdder(5);
const add10 = makeAdder(10);
 
console.log(add5(2));
console.log(add10(2));
 
$ node closures.js
jsbyexample
7
12

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