Editing Foreach loop (section)

=== JavaScript ===
In [[ECMAScript 5]], a callback-based <code>forEach()</code> method was added to the array [[Prototype-based programming|prototype]]:<ref>{{Cite web |date=2024-07-25 |title=Array.prototype.forEach() - JavaScript {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach |access-date=2024-12-03 |website=developer.mozilla.org |language=en-US}}</ref><syntaxhighlight lang="javascript">
myArray.forEach(function (item, index) {
    // Do stuff with item and index 
    // The index variable can be omitted from the parameter list if not needed
});
</syntaxhighlight>The [[ECMAScript 6]] standard introduced a more conventional <code>[https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/for...of for..of]</code> syntax that works on all [[Iterator|iterables]] rather than operating on only array instances. However, no index variable is available with the syntax.
<syntaxhighlight lang="javascript">
for (const item of myArray) {
    // Do stuff with item
}
</syntaxhighlight>

For unordered iteration over the keys in an object, [[JavaScript]] features the <code>for..in</code> loop:

<syntaxhighlight lang="javascript">
for (const key in myObject) {
    // Do stuff with myObject[key]
}
</syntaxhighlight>

To limit the iteration to the object's own properties, excluding those inherited through the prototype chain, it's often useful to add a <code>hasOwnProperty()</code> test (or a <code>hasOwn()</code> test if supported).<ref>{{Cite web |date=2023-09-25 |title=Object.hasOwn() - JavaScript {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#description |access-date=2024-12-03 |website=developer.mozilla.org |language=en-US}}</ref>

<syntaxhighlight lang="javascript">for (const key in myObject) {
    // Available in older browsers
    if (myObject.hasOwnProperty(key)) {
        // Do stuff with object[key]
    }
    // Preferred in modern browsers
    if (Object.hasOwn(myObject, key)) {
        // Do stuff with object[key]
    }
}</syntaxhighlight>

Alternatively, the <code>Object.keys()</code> method combined with the <code>for..of</code> loop can be used for a less verbose way to iterate over the keys of an object.<ref>{{cite web | url= https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys | title=Object.keys | work=Mozilla Developer Network | access-date=May 7, 2014}}</ref>
<syntaxhighlight lang="javascript">
const book = { 
    name: "A Christmas Carol", 
    author: "Charles Dickens" 
}; 
for (const key of Object.keys(book)) {
    console.log(`Key: ${key}, Value: ${book[key]}`);
}
</syntaxhighlight>