🔍 Understanding `Array.includes()` in JavaScript

Hey developers 👋,

Just another casual day discussing code with my colleagues when one of them brought up something surprisingly interesting — the good old includes() method in JavaScript arrays. Most of us use it regularly, but have you ever paused and asked:

"What really happens inside includes()?"

Let’s explore how it works, including edge cases and the comparison logic behind it.

🧠 What is `Array.prototype.includes()`?

The includes() method determines whether an array contains a specified element. It returns a boolean: true if the element is found, false otherwise.

✅ Syntax:

array.includes(searchElement)
array.includes(searchElement, fromIndex)

searchElement: The value to search for.
fromIndex (optional): The position in the array at which to start the search. Defaults to 0.

📦 Example: Simple Usage

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', 'vasanth', 'alan'];
console.log(nameList.includes('joyal')); // Output: true

⚙️ How It Works Internally

Let’s dive into different scenarios and break them down.

🔁 Case 1: Using without `fromIndex`

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', 'vasanth', 'alan'];
console.log(nameList.includes('ayoob')); // Output: true

What happens:

nameList.length = 6
fromIndex = undefined
The method begins checking from index 0 because the fromIndex is undefined
'ayoob' is found at index 2 → returns true

🔁 Case 2: Using `fromIndex` Within Array Length

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', 'vasanth', 'alan'];
console.log(nameList.includes('ayoob', 1)); // Output: true

What happens:

nameList.length = 6
fromIndex = 1
The method skips index 0 and begins checking from index 1
'ayoob' is found at index 2 → returns true

🚫 Case 3: `fromIndex` Exceeds Array Length

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', 'vasanth', 'alan'];
console.log(nameList.includes('ayoob', 10)); // Output: false

Why?

If fromIndex > nameList.length, the method short-circuits and returns false immediately. The array isn’t even searched.

➖ Case 4: Negative `fromIndex`

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', 'vasanth', 'alan'];
console.log(nameList.includes('ayoob', -1)); // Output: false

What’s going on:

A negative index is interpreted as array.length + fromIndex
In this case: 6 + (-1) = 5, so search starts at index 5
'ayoob' is at index 2, so it’s missed → returns false

🤯 Case 5: Searching for `NaN`

const nameList = ['ram', 'joyal', 'ayoob', 'abdul', NaN, 'alan'];
console.log(nameList.includes(NaN)); // Output: true

Wait, what?!
You might expect this to return false because:

NaN === NaN // false

But includes() uses the SameValueZero algorithm, which treats NaN as equal to NaN.

🔬 SameValueZero Comparison

Here’s a simplified version of how SameValueZero works:

function SameValueZero(x, y) {
  return x === y || (x !== x && y !== y); // true for NaN === NaN
}

🔄 Comparison Table

Value A	Value B	`==` Result	`===`Result	`SameValueZero`Result
`1`	`1`	✅ `true`	✅ `true`	✅ `true`
`1`	`'1'`	✅ `true`	❌ `false`	❌ `false`
`'a'`	`'a'`	✅ `true`	✅ `true`	✅ `true`
`0`	`-0`	✅ `true`	✅ `true`	✅ `true`
`NaN`	`NaN`	❌ `false`	❌ `false`	✅ `true`
`undefined`	`null`	❌ `false`	❌ `false`	❌ `false`

Important note: +0 and -0 are treated as equal in both === and SameValueZero.

🧵 Final Thoughts

Next time you're debugging a subtle bug with includes(), remember: it’s not just a check — it's powered by SameValueZero, and how you set fromIndex matters.

If you found this helpful, drop a ❤️ or leave a comment. Happy coding!

✍️ Author: Vetriselvan

👨‍💻 Frontend Developer | Code Lover | Exploring Angular’s future

vetriselvan Panneerselvam @vetriselvan_11