The Strangest Equality in JavaScript — Explained with Real Examples

🧠 Why [] == ![] is true and other JavaScript WTFs 🤯

If you've ever stared at a JavaScript comparison and thought,

"This can't be right..."
You're not alone.

JavaScript’s type coercion is infamous for producing results that seem straight-up wrong — until you understand what’s really going on under the hood.

Let’s walk through some of the quirkiest examples — and break them down step by step.

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🧪 Example 1: [] == ![] // true

Wait, what?!
An empty array is equal to its own negation?

Let’s break it down:

[] == ![]

Step-by-step:

1. ![] → First, ![] is evaluated.

   • An empty array is truthy.

   • So ![] becomes false.

2. Now we're comparing: [] == false

3. JavaScript converts the array to a primitive.

   • [] → '' (empty string)

4. Then it converts both sides to numbers:

    '' == false
    0 == 0 // true
   

👉 Final result: true

Wild, right?

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🧪 Example 2: [] == [] // false

This one feels even more confusing:

[] == [] // false

Why false?

Because JavaScript compares references when using == (or ===) for objects and arrays.

const a = [];
const b = [];

a == b // false — different memory locations

👉 Even if the contents are the same, they’re different objects.

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🧪 Example 3: {} + [] → 0

{} + [] // 0

This looks like it should concatenate an object and an array, right?

But here's the trick:
When this runs in the browser (outside a block), the {} is interpreted as an empty block, not an object.

So it becomes:

+[] // 0

The unary + converts [] to a number:

Number([]) → 0

👉 Final result: 0

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🧪 Example 4: [] + {} → "[object Object]"

Here’s one that actually is string concatenation:

[] + {} // "[object Object]"

How?

1. [] becomes '' (empty string)

2. {} becomes '[object Object]'

3. Concatenation gives:

    '' + '[object Object]' → '[object Object]'
   

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🎯 TL;DR — What to Remember

• == does type coercion — things are often converted to numbers or strings before comparison.

• Arrays/objects are compared by reference, not value.

• + can mean numeric addition or string concatenation depending on context.

• Always use === unless you really know what you're doing.

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💡 Pro Tip:

If a comparison feels weird, try breaking it down like the engine does.
Or better yet — avoid == altogether unless you need type coercion magic.

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💬 Bonus

What’s the weirdest equality case you’ve seen in JavaScript?

Drop your favorite WTF JS comparison in the comments 👇

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