JavaScript Loops: A Comprehensive Guide

JavaScript Loops: A Comprehensive Guide

Objects, arrays, and strings — for, forEach, for…of, for…in, includes, while, do…while, every, some

Have you ever wondered what loop to use when coding JavaScript? Sometimes you may wonder if there is a shortcut that will save you time, or if your loop is going to cause a bug.

The objective of this article is to put all the important loop information in one place so that you can just scroll through and find your loop.

So, without further ado, here are the JavaScript loops you can use in your code, organized into loops for Objects, Arrays, and Strings.

Loop Through Objects

1. for…in

for…in is the JavaScript loop best suited for objects. This loop iterates over all the properties of an object. The keys of the properties must be strings. Keys that are symbols are skipped.

The object’s properties must be enumerable properties if you need them to be seen by the for…in loop. Enumerable properties are just properties that have an internal enumerable flag set to true. Objects created with an object literal or via a property initializer (new Object()Object.create()) will have the enumerable property set to true by default.

If you don’t want the for…in loop to see a certain property, you can set a nonenumerable property like this:

const myObject = {
    keyOne: 5,
    keyTwo: "valueTwo",
    differentkey: "value3"
};
Object.defineProperty(myObject, 'newKey', {
  value: "can't see me in a for...in loop",
  configurable: false, // default is false
  writable: false, // default is false
  enumerable: false, // default is false
});

The for…in loop iterates through an object in a random order, so developers shouldn’t rely on the order in which the properties were added to the object.

It is important to note the following when modifying/adding/deleting properties of an object while still iterating through the loop:

  • Any deleted property may be visited before it is deleted.
  • Any modified property may or may not be visited before it is modified.
  • An added property may or may not be visited.

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