Switching my language from Ruby to JavaScript left me in such a confusing stage, especially the difference in syntax. One of my most used iteration methods is the loop, which is available in both ruby and JS. Loops offer a quick and easy way to skim through iterable Objects. There are several statements for loops (for statement, do...while statement, while statement, break statement, etc). Today, I'll discuss for, for...in and for...of statements usage. This blog should give you an overview of how to use each statement correctly and effectively.

1. for Statement:

The most common for loop and easiest to use in my opinion. A for loop executes the same block of code until a specified condition evaluates to false.

Syntax:

for ([initialExpression]; [conditionExpression]; [incrementExpression]){
  //code block to be executed
}

There are three main expressions in a for loop:

• initialExpression:
  initialExpression initializes a counter variable, typically by using var or let. Variables being declared using let are local to the statement. initialExpression is executed (one time) before the execution of the code block and its result will be discarded.

• conditionExpression:
  conditionExpression is evaluated to decide if the code block is being executed. If this expression evaluated to be true, the for loop will continue, the code block will be executed. If the result returns false, the for loop terminates.

• incrementExpression:
  Typically used to increase or adjust the counter variable before another loop is executed. To execute multiple statements, use a block statement {...} to group those statements.

When a for loop is executed, these following steps occur:

Example:

In order to return an array of only even numbers, we can use a for loop:

const num = [1, 5, 7, 8, 9, 10]
let even = []

for(let i = 0; i < num.length; i++) {
    if (num[i] % 2 === 0) {
        even.push(num[i])
    }
}
return even
//[ 8, 10 ]

So what's happening in our for loop?

1. We initialize i as a counter variable starting from 0.
2. Condition is being evaluated. In this case, num.length is 6 so as long as i's value is less than 6, our for loop continues.
3. Our code block is executed since the condition is evaluated to be true.
4. Counter variable being incremented before the next for loop start, which brings i's value to 1. Then back to step 2 until i's value = 6 and the condition is evaluated to be false. The loop terminates.
5. Return the even array.

2. For..in Statement:

A handy for loop that is used to loop over enumerables (JSON objects, a bag of Skittles are enumerables). Not recommended to be used with arrays where index order is important.

Syntax:

for(let key in object) {
  //code block to be executed
}

Our object looks like this:

let object = {
    key: value
    ...
}

or

object.key = value

Crucial parameters:

• key : get assigned by properties of the object each time a for loop is executed.
• object : a specified object that non-symbol properties will get iterated over.

Example:

• Ability to iterate over an object's property and access the property's value:

const profile = {
    first_name: 'Jane',
    last_name: 'Sparkling',
    sex: 'F',
    dob: '05/23/1968'
};

for(let p in profile) {
    console.log(p + ': ' + profile[p])
}

On console:

first_name: Jane
last_name: Sparkling
sex: F
dob: 05/23/1968

What's happening?

1. We declare p (as properties) of our profile Object. So in this case, p can be first_name/last_name/sex/dob in our loop.
2. When we console.log our result, p will only return the Object's property, so in order to access the value, we have to call profile[p] to get that property's value.

• Iterate through an array where the element's index is not important. for..in loop only returns truthy values:

let arr = []
arr[1] = 3
arr[2] = 4
arr[4] = 5
//empty, 3, 4, empty, 5

for(num in arr){
    console.log(arr[num])
}

Result:

3
4
5

If we used a for loop:

for (let i = 0; i < arr.length; i++) {
    console.log(arr[i]);    
}

The result will also include falsy values:

undefined
3
4
undefined
5

3. For..of Statement:

for..of will loop through values of an iterable object (JSON array, a line in DMV are iterables). Can be used to handle arrays, DOM collection, etc.

Syntax:

for(let variable of iterable) {
  //code block to be executed
}

Crucial parameters:

• variable: On each iteration, a property's value will be assigned to a variable. A variable can be declared by using let, const or var.
• iterable: An iterable Object.

Example:

• Iterating over an array:

const nums = [21, 17, 18, 22, 30]

for(const num of nums){
    console.log(num)
}
//21, 17, 18, 22, 30

or you can also use let to declare a variable if you need to change its value later:

const nums = [21, 17, 18, 22, 30]

for(let num of nums){
    num *= 2
    console.log(num)
}
//42, 34, 36, 44, 60

• Iterating over a DOM Collection to add a tag with image URL fetched from an API:

//json["message"] is an array of image URL
for(const img of json["message"]) {
    let pic = document.createElement('img')
    pic.setAttribute('src', img)
    image.appendChild(pic)
}
// in HTML file:
//<img src="https://images.dog.ceo/breeds/dane-great/n02109047_1672.jpg">

• Iterating over a string:

const iterable = 'boo';

for (const value of iterable) {
  console.log(value);
}
//b
//o
//o

4. for..in and for..of comparison:

Both for..in and for..of iterates over objects, but the main difference is the way it iterates:

Example from for...of - javascript | MDN

const iterable = [3, 5, 7];
iterable.foo = 'hello';
//[3, 5, 7, foo: "hello"]

for (const i in iterable) {
  console.log(i); 
}
// logs 0, 1, 2, foo

The for..in loop iterates over enumerable properties of an object. So the reason it doesn't log 3, 5, 7 or hello because these are not enumerable properties, but in fact, they are values. It logs array indexes, which are enumerable properties with integer names.

const iterable = [3, 5, 7];
iterable.foo = 'hello';
//[3, 5, 7, foo: "hello"]

for (const i of iterable) {
  console.log(i);
}
// logs 3, 5, 7

The for..of loop will iterate over properties and logs its values. Return variables will only be the value and none of the properties.

Bonus:
In order to log all properties' values:

const iterable = [3, 5, 7];
iterable.foo = 'hello';
//[3, 5, 7, foo: "hello"]

for (const i in iterable) {
  console.log(iterable[i]); 
}
// logs 3, 5, 7, hello

Helpful Resources:

• for..in
• for..of
• loops and iterations
• A deeper comparison between for..in and for..of here