for...of - JavaScript
the for...of statement creates a loop iterating over iterable objects, including: built-in string, array, array-like objects (e.g., arguments or nodelist), typedarray, map, set, and user-defined iterables.
...lections like nodelist: the following example adds a read class to paragraphs that are direct descendants of an article: // note: this will only work in platforms that have // implemented nodelist.prototype[symbol.iterator] const articleparagraphs = document.queryselectorall('article > p'); for (const paragraph of articleparagraphs) { paragraph.classlist.add('read'); } closing iterators in for...of loops, abrupt iteration termination can be caused by break, throw or return.
...functions generating an iterable object: function* fibonacci() { // a generator function let [prev, curr] = [0, 1]; while (true) { [prev, curr] = [curr, prev + curr]; yield curr; } } for (const n of fibonacci()) { console.log(n); // truncate the sequence at 1000 if (n >= 1000) { break; } } do not reuse generators generators should not be re-used, even if the for...of loop is terminated early, for example via the break keyword.
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Warning: JavaScript 1.6's for-each-in loops are deprecated - JavaScript
for each (var x in object) { console.log(x); // 10 // 20 } alternative standard syntax you can now use the standard for...in loop to iterate over specified object keys, and get each value inside the loop: var object = { a: 10, b: 20 }; for (var key in object) { var x = object[key]; console.log(x); // 10 // 20 } or, using for...of (es2015) and object.values (es2017), you can get an array of the specified object values and iterate over the array like this: var object = { a: 10, b: 20 }; for (var x of object.values(object)) { console.log(x); // 10 // 20 } array iteration for each...in has been used to iterate over specified array elements.
... deprecated syntax var array = [10, 20, 30]; for each (var x in array) { console.log(x); // 10 // 20 // 30 } alternative standard syntax this is now possible with for...of (es2015) loops as well.
... var array = [10, 20, 30]; for (var x of array) { console.log(x); // 10 // 20 // 30 } iterating over a null-able array for each...in does nothing if the specified value is null or undefined, but for...of will throw an exception in these cases.
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Loops and iteration - JavaScript
the statements for loops provided in javascript are: for statement do...while statement while statement labeled statement break statement continue statement for...in statement for...of statement for statement a for loop repeats until a specified condition evaluates to false.
... for...of statement the for...of statement creates a loop iterating over iterable objects (including array, map, set, arguments object and so on), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
... for (variable of object) statement the following example shows the difference between a for...of loop and a for...in loop.
... while for...in iterates over property names, for...of iterates over property values: const arr = [3, 5, 7]; arr.foo = 'hello'; for (let i in arr) { console.log(i); // logs "0", "1", "2", "foo" } for (let i of arr) { console.log(i); // logs 3, 5, 7 } « previousnext » ...

ECMAScript 2015 support in Mozilla - Archive of obsolete content
standard library additions to the array object array iteration with for...of (firefox 13) array.from() (firefox 32) array.of() (firefox 25) array.prototype.fill() (firefox 31) array.prototype.find(), array.prototype.findindex() (firefox 25) array.prototype.entries(), array.prototype.keys() (firefox 28), array.prototype.values() array.prototype.copywithin() (firefox 32) get array[@@species] (firefox 48) new map and set objects, and their weak counterparts m...
...ap (firefox 13) map iteration with for...of (firefox 17) map.prototype.foreach() (firefox 25) map.prototype.entries() (firefox 20) map.prototype.keys() (firefox 20) map.prototype.values() constructor argument: new map(null) (firefox 37) monkey-patched set() in constructor (firefox 37) get map[@@species] (firefox 41) set (firefox 13) set iteration with for...of (firefox 17) set.prototype.foreach() (firefox 25) set.prototype.entries(), set.prototype.keys(), set.prototype.values() (firefox 24) constructor argument: new set(null) (firefox 37) monkey-patched add() in constructor (firefox 37) get set[@@species] (firefox 41) weakmap (firefox 6) weakmap.clear() (firefox 20) optional iterable argument in weakmap constructor (fire...
...iterator property (firefox 36) spread operator for function calls (firefox 27) use symbol.iterator property (firefox 36) const (js 1.5, firefox 1.0) (es2015 compliance bug 950547 implemented in firefox 51) let (js 1.7, firefox 2) (es2015 compliance bug 950547 implemented in firefox 51) destructuring assignment (js 1.7, firefox 2) (es2015 compliance bug 1055984) statements for...of (firefox 13) works in terms of .iterator() and .next() (firefox 17) use "@@iterator" property (firefox 27) use symbol.iterator property (firefox 36) functions rest parameters (firefox 15) default parameters (firefox 15) parameters without defaults after default parameters (firefox 26) destructured parameters with default value assignment (firefox 41) arrow f...

A re-introduction to JavaScript (JS tutorial) - JavaScript
for (var i = 0; i < 5; i++) { // will execute 5 times } javascript also contains two other prominent for loops: for...of for (let value of array) { // do something with value } and for...in: for (let property in object) { // do something with object property } the && and || operators use short-circuit logic, which means whether they will execute their second operand is dependent on the first.
... if you query a non-existent array index, you'll get a value of undefined in return: typeof a[90]; // undefined if you take the above about [] and length into account, you can iterate over an array using the following for loop: for (var i = 0; i < a.length; i++) { // do something with a[i] } es2015 introduced the more concise for...of loop for iterable objects such as arrays: for (const currentvalue of a) { // do something with currentvalue } you could also iterate over an array using a for...in loop, however this does not iterate over the array elements, but the array indices.
...we will also replace the for loop with a for...of loop to return the values within our variable.

Iterators and generators - JavaScript
iterators and generators bring the concept of iteration directly into the core language and provide a mechanism for customizing the behavior of for...of loops.
... for details, see also: iteration_protocols for...of function* and generator yield and yield* iterators in javascript an iterator is an object which defines a sequence and potentially a return value upon its termination.
... function* makerangeiterator(start = 0, end = 100, step = 1) { let iterationcount = 0; for (let i = start; i < end; i += step) { iterationcount++; yield i; } return iterationcount; } iterables an object is iterable if it defines its iteration behavior, such as what values are looped over in a for...of construct.

Iteration protocols - JavaScript
the iterable protocol the iterable protocol allows javascript objects to define or customize their iteration behavior, such as what values are looped over in a for...of construct.
... whenever an object needs to be iterated (such as at the beginning of a for...of loop), its @@iterator method is called with no arguments, and the returned iterator is used to obtain the values to be iterated.
...[{}, 'c'] ]).get(myobj); // "b" new set([1, 2, 3]).has(3); // true new set('123').has('2'); // true new weakset(function* () { yield {} yield myobj yield {} }()).has(myobj); // true see also promise.all(iterable) promise.race(iterable) array.from(iterable) syntaxes expecting iterables some statements and expressions expect iterables, for example the for...of loops, the spread operator), yield*, and destructuring assignment: for (let value of ['a', 'b', 'c']) { console.log(value); } // "a" // "b" // "c" console.log([...'abc']); // ["a", "b", "c"] function* gen() { yield* ['a', 'b', 'c']; } console.log(gen().next()); // { value: "a", done: false } [a, b, c] = new set(['a', 'b', 'c']); console.log(a); // "a" non-well-formed it...

Deprecated and obsolete features - JavaScript
use for...of instead.
...use for...of instead.

SyntaxError: a declaration in the head of a for-of loop can't have an initializer - JavaScript
the javascript exception "a declaration in the head of a for-of loop can't have an initializer" occurs when the head of a for...of loop contains an initializer expression such as |for (var i = 0 of iterable)|.
... the head of a for...of loop contains an initializer expression.

for...in - JavaScript
therefore, it is better to use a for loop with a numeric index (or array.prototype.foreach() or the for...of loop) when iterating over arrays where the order of access is important.
... given that for...in is built for iterating object properties, not recommended for use with arrays, and options like array.prototype.foreach() and for...of exist, what might be the use of for...in at all?

Array comprehensions - Archive of obsolete content
syntax [for (x of iterable) x] [for (x of iterable) if (condition) x] [for (x of iterable) for (y of iterable) x + y] description inside array comprehensions, these two kinds of components are allowed: for...of and if the for-of iteration is always the first component.

Generator comprehensions - Archive of obsolete content
syntax (for (x of iterable) x) (for (x of iterable) if (condition) x) (for (x of iterable) for (y of iterable) x + y) description inside generator comprehensions, these two kinds of components are allowed: for...of and if the for-of iteration is always the first component.

for each...in - Archive of obsolete content
consider using for...of instead.

CSSStyleSheet.cssRules - Web APIs
examples individual rules within the stylesheet can then be accessed by index: let rulelist = document.stylesheets[0].cssrules; for (let i=0; i < rulelist.length; i++) { processrule(rulelist[i]); } rules can also be accessed using for...of: let rulelist = document.stylesheets[0].cssrules; for (let rule of rulelist) { processrule(rule); } however, because cssrule is not a proper array, you can't use foreach().

DOMTokenList.entries() - Web APIs
we when retrieve an iterator containing the key/value pairs using entries(), then iterate through each one using a for...of loop, writing them to the <span>'s node.textcontent.