How do JavaScript closures work?

Asked : Nov 17

Viewed : 19 times

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables, and the like), but who does not understand closures themselves?

I have seen the Scheme example given on Wikipedia, but unfortunately, it did not help.

variables javascript function scope closures 

Nov 17

2 Answers

A closure is a pairing of:

  1. A function, and
  2. A reference to that function's outer scope (lexical environment)

A lexical environment is part of every execution context (stack frame) and is a map between identifiers (ie. local variable names) and values.

Every function in JavaScript maintains a reference to its outer lexical environment. This reference is used to configure the execution context created when a function is invoked. This reference enables code inside the function to "see" variables declared outside the function, regardless of when and where the function is called.

If a function was called by a function, which in turn was called by another function, then a chain of references to outer lexical environments is created. This chain is called the scope chain.

In the following code, inner forms a closure with the lexical environment of the execution context created when foo is invoked, closing over variable secret:

function foo() {
  const secret = Math.trunc(Math.random()*100)
  return function inner() {
    console.log(`The secret number is ${secret}.`)
  }
}
const f = foo() // `secret` is not directly accessible from outside `foo`
f() // The only way to retrieve `secret`, is to invoke `f`

A closure is a pairing of:

  1. A function, and
  2. A reference to that function's outer scope (lexical environment)

A lexical environment is part of every execution context (stack frame) and is a map between identifiers (ie. local variable names) and values.

Every function in JavaScript maintains a reference to its outer lexical environment. This reference is used to configure the execution context created when a function is invoked. This reference enables code inside the function to "see" variables declared outside the function, regardless of when and where the function is called.

If a function was called by a function, which in turn was called by another function, then a chain of references to outer lexical environments is created. This chain is called the scope chain.

In the following code, inner forms a closure with the lexical environment of the execution context created when foo is invoked, closing over variable secret:

function foo() {
  const secret = Math.trunc(Math.random()*100)
  return function inner() {
    console.log(`The secret number is ${secret}.`)
  }
}
const f = foo() // `secret` is not directly accessible from outside `foo`
f() // The only way to retrieve `secret`, is to invoke `f`

In other words: in JavaScript, functions carry a reference to a private "box of state", to which only they (and any other functions declared within the same lexical environment) have access. This box of the state is invisible to the caller of the function, delivering an excellent mechanism for data-hiding and encapsulation.

And remember: functions in JavaScript can be passed around like variables (first-class functions), meaning these pairings of functionality and state can be passed around your program: similar to how you might pass an instance of a class around in C++.

If JavaScript did not have closures, then more states would have to be passed between functions explicitly, making parameter lists longer and code noisier.

So, if you want a function to always have access to a private piece of state, you can use a closure.

...and frequently we do want to associate the state with a function. For example, in Java or C++, when you add a private instance variable and a method to a class, you are associating the state with functionality.

In C and most other common languages, after a function returns, all the local variables are no longer accessible because the stack-frame is destroyed. In JavaScript, if you declare a function within another function, then the local variables of the outer function can remain accessible after returning from it. In this way, in the code above, secret remains available to the function object inner, after it has been returned from foo.

Uses of Closures

Closures are useful whenever you need a private state associated with a function. This is a very common scenario - and remember: JavaScript did not have a class syntax until 2015, and it still does not have a private field syntax. Closures meet this need.

Private Instance Variables

In the following code, the function toString closes over the details of the car.

function Car(manufacturer, model, year, color) {
  return {
    toString() {
      return `${manufacturer} ${model} (${year}, ${color})`
    }
  }
}
const car = new Car('Aston Martin','V8 Vantage','2012','Quantum Silver')
console.log(car.toString())

Functional Programming

In the following code, the function inner closes over both fn and args.

function curry(fn) {
  const args = []
  return function inner(arg) {
    if(args.length === fn.length) return fn(...args)
    args.push(arg)
    return inner
  }
}

function add(a, b) {
  return a + b
}

const curriedAdd = curry(add)
console.log(curriedAdd(2)(3)()) // 5

Event-Oriented Programming

In the following code, function onClick closes over variable BACKGROUND_COLOR.

const $ = document.querySelector.bind(document)
const BACKGROUND_COLOR = 'rgba(200,200,242,1)'

function onClick() {
  $('body').style.background = BACKGROUND_COLOR
}

$('button').addEventListener('click', onClick)
<button>Set background color</button>

Modularization

In the following example, all the implementation details are hidden inside an immediately executed function expression. The functions tick and toString close over the private state and functions, they need to complete their work. Closures have enabled us to modularise and encapsulate our code.

 

let namespace = {};

(function foo(n) {
  let numbers = []
  function format(n) {
    return Math.trunc(n)
  }
  function tick() {
    numbers.push(Math.random() * 100)
  }
  function toString() {
    return numbers.map(format)
  }
  n.counter = {
    tick,
    toString
  }
}(namespace))

const counter = namespace.counter
counter.tick()
counter.tick()
console.log(counter.toString())

Examples

Example 1

This example shows that the local variables are not copied in the closure: the closure maintains a reference to the original variables themselves. It is as though the stack-frame stays alive in memory even after the outer function exits.

function foo() {
  let x = 42
  let inner  = function() { console.log(x) }
  x = x+1
  return inner
}
var f = foo()
f() // logs 43

Example 2

In the following code, three methods log, increment, and update all close over the same lexical environment.

And every time createObject is called, a new execution context (stack frame) is created and a completely new variable x, and a new set of functions (log etc.) are created, that close over this new variable.

 

function createObject() {
  let x = 42;
  return {
    log() { console.log(x) },
    increment() { x++ },
    update(value) { x = value }
  }
}

const o = createObject()
o.increment()
o.log() // 43
o.update(5)
o.log() // 5
const p = createObject()
p.log() // 42

Example 3

If you are using variables declared using,  var  be careful you understand which variable you are closing over. Variables declared using var are hoisted. This is much less of a problem in modern JavaScript due to the introduction of let and const.

In the following code, each time around the loop, a new function inner is created, which closes over i. But because var i is hoisted outside the loop, all of these inner functions close over the same variable, meaning that the final value of i (3) is printed, three times.

 

function foo() {
  var result = []
  for (var i = 0; i < 3; i++) {
    result.push(function inner() { console.log(i) } )
  }
  return result
}

const result = foo()
// The following will print `3`, three times...
for (var i = 0; i < 3; i++) {
  result[i]() 
}

Final points:

  • Whenever a function is declared in JavaScript closure is created.
  • Returning a function from inside another function is the classic example of closure, because the state inside the outer function is implicitly available to the returned inner function, even after the outer function has completed execution.
  • Whenever you use eval() inside a function, a closure is used. In the text you eval can reference local variables of the function, and in the non-strict mode, you can even create new local variables by using eval('var foo = …').
  • When you use new Function(…) (the Function constructor) inside a function, it does not close over its lexical environment: it closes over the global context instead. The new function cannot reference the local variables of the outer function.
  • A closure in JavaScript is like keeping a reference (NOT a copy) to the scope at the point of function declaration, which in turn keeps a reference to its outer scope, and so on, all the way to the global object at the top of the scope chain.
  • A closure is created when a function is declared; this closure is used to configure the execution context when the function is invoked.
  • A new set of local variables is created every time a function is called.

Links

  • Douglas Crockford simulated private attributes and private methods for an object, using closures.
  • A great explanation of how closures can cause memory leaks in IE if you are not careful.
  • MDN documentation on JavaScript Closures.

answered Dec 31


cases for closures

Currying

Function currying is another powerful concept in functional programming. To implement a curried function in JavaScript, you would use closures.

Currying a function can be described as transforming a function and is executed like this: add(1, 2, 3) to add(1)(2)(3).

function add(a) { 
  return function(b) {
    return function(c) {
      return a + b + c;
    };
  };
};

add(1)(2)(3) // returns 6

The add the function takes a single argument and then returns two functions that are nested in one after the other. The goal of currying is to take a bunch of arguments and eventually end up with a single value.

Higher-order functions

The goal of a higher-order function is to take a function as an argument and return a result. Array methods such as map and reduce are examples of higher-order functions.

const arrayOfNumbers = [1, 2, 3];
const displayNumber = (num) => {
  console.log(num);
}
arrayOfNumbers.forEach(displayNumber)

The Array.prototype.forEach the higher-order function here accepts displayNumber as an argument and then executes it for each element in the arrayOfNumbers. If you’ve used a UI framework such as Vue or React, you might be familiar with higher-order components, which are essentially the same thing as higher-order functions.

So what’s the difference is between higher-order functions and currying? While a higher-order function takes a function as an argument returns a value, a curried function returns a function as a result, which eventually leads to a value.

DOM element managers

This is a common design pattern often used to get and set properties of DOM elements. In the following example, we’ll make an element manager to style elements.

function makeStyleManager(selector) {
    const element = document.querySelector(selector);
    const currentStyles = {...window.getComputedStyle(element)};
 
    return {
        getStyle: function(CSSproperty) {
            return currentStyles[CSSproperty];
        },
        setStyle: function(CSSproperty, newStyle) {
            element.style[CSSproperty] = newStyle;
        },
    };
};

const bodyStyleManager = makeStyleManager('body');
bodyStyleManager.getStyle('background-color'); // returns rgb(0,0,0)
bodyStyleManager.setStyle('background-color', 'red'); // sets bg color to red

makeStyleManager returns an object that gives access to two functions, which are part of a closure alongside the element and currentStyles variables. Even after makeStyleManager has finished executing, the getStyle and setStyle functions have access to the variables.

answered Dec 31


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