In Java you can use a for
loop to traverse objects in an array as follows:
String[] myStringArray = {"Hello", "World"};
for (String s : myStringArray)
{
// Do something
}
Can you do the same in JavaScript?
arraysfor-loopjavascriptloops
In Java you can use a for
loop to traverse objects in an array as follows:
String[] myStringArray = {"Hello", "World"};
for (String s : myStringArray)
{
// Do something
}
Can you do the same in JavaScript?
A closure is a pairing of:
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 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
.
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.
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())
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
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>
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())
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
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
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]()
}
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.eval()
inside a function, a closure is used. 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 = …')
.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.Modern browsers have Array#includes
, which does exactly that and is widely supported by everyone except IE:
console.log(['joe', 'jane', 'mary'].includes('jane')); //true
You can also use Array#indexOf
, which is less direct, but doesn't require polyfills for outdated browsers.
console.log(['joe', 'jane', 'mary'].indexOf('jane') >= 0); //true
Many frameworks also offer similar methods:
$.inArray(value, array, [fromIndex])
_.contains(array, value)
(also aliased as _.include
and _.includes
)dojo.indexOf(array, value, [fromIndex, findLast])
array.indexOf(value)
array.indexOf(value)
findValue(array, value)
array.indexOf(value)
Ext.Array.contains(array, value)
_.includes(array, value, [from])
(is _.contains
prior 4.0.0)R.includes(value, array)
Notice that some frameworks implement this as a function, while others add the function to the array prototype.
Best Answer
Three main options:
for (var i = 0; i < xs.length; i++) { console.log(xs[i]); }
xs.forEach((x, i) => console.log(x));
for (const x of xs) { console.log(x); }
Detailed examples are below.
1. Sequential
for
loop:Pros
break
andcontinue
flow control statementsCons
2.
Array.prototype.forEach
:The ES5 specification introduced a lot of beneficial array methods. One of them, the
Array.prototype.forEach
, gave us a concise way to iterate over an array:Being almost ten years as the time of writing that the ES5 specification was released (Dec. 2009), it has been implemented by nearly all modern engines in the desktop, server, and mobile environments, so it's safe to use them.
And with the ES6 arrow function syntax, it's even more succinct:
Arrow functions are also widely implemented unless you plan to support ancient platforms (e.g., Internet Explorer 11); you are also safe to go.
Pros
Cons
break
/continue
Normally, you can replace the need to
break
out of imperative loops by filtering the array elements before iterating them, for example:Keep in mind if you are iterating an array to build another array from it, you should use
map
. I've seen this anti-pattern so many times.Anti-pattern:
Proper use case of map:
Also, if you are trying to reduce the array to a value, for example, you want to sum an array of numbers, you should use the reduce method.
Anti-pattern:
Proper use of reduce:
3. ES6
for-of
statement:The ES6 standard introduces the concept of iterable objects and defines a new construct for traversing data, the
for...of
statement.This statement works for any kind of iterable object and also for generators (any object that has a
\[Symbol.iterator\]
property).Array objects are by definition built-in iterables in ES6, so you can use this statement on them:
Pros
break
/continue
).Cons
Do not use
for...in
@zipcodeman suggests the use of the
for...in
statement, but for iterating arraysfor-in
should be avoided, that statement is meant to enumerate object properties.It shouldn't be used for array-like objects because:
The second point is that it can give you a lot of problems, for example, if you extend the
Array.prototype
object to include a method there, that property will also be enumerated.For example:
The above code will console log "a", "b", "c", and "foo!".
That can be particularly a problem if you use some library that relies heavily on native prototypes augmentation (such as MooTools).
The
for-in
statement, as I said before, is there to enumerate object properties, for example:In the above example, the
hasOwnProperty
method allows you to enumerate only own properties. That's it, only the properties that the object physically has, no inherited properties.I would recommend you to read the following article: