Back to basics: Why Does {} + [] Return 0?

Most of us working regularly with JavaScript are already more or less accustomed to some of the unexpected, quirky behavior it exhibits.

Take this classic example:

3 + "3" // "33"

Here’s one I stumbled across that really made me pause:

{} + [] // 0

Take a wild guess why this happens.

It’s because {} is not treated as an object literal here — instead, it’s interpreted as an empty code block. That leaves us with:

+[] // => 0

The unary + coerces the empty array into a number, and the result is 0.

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🧩 More Examples of Type Coercion

The Stack Overflow answer I found also mentions:

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

Here, [] is treated as an empty string, and {} is treated as an object literal. JavaScript tries to concatenate them as strings — hence: "[object Object]".

How about:

[] + [] // ""

Straightforward, right? Both arrays are treated as empty strings. Concatenating two empty strings gives… an empty string.

And then there's this one:

{} + {} // NaN

This one’s a little trickier. The first {} is still treated as a code block. The second {} is coerced using the + operator. Since objects can’t be coerced into numbers, the result is NaN.

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⚙️ Why It Matters in Real Code

I know, nobody is adding arrays and objects. This stuff is mostly a part of JS trivia.

But I actually stumbled into something similar that led me down the "type and coercion" rabbit hole.

const pl = // some complex formula with no type safety

if (pl) {
  // update UI
}

The bug? Sometimes pl was 0, and the block didn’t execute — because 0 is falsy in JavaScript.

At first, the condition seemed intended to guard against undefined or null. So we tried a safer check:

if (typeof pl === "number") {
  // update UI
}

Pretty solid — except for one catch:

typeof NaN // "number"

Yep. NaN is of type "number".

So this condition passed even when pl was NaN, and the UI still updated with invalid values.

We ended up adding a combination of checks (like !isNaN(pl)) and some stronger type safety upstream.

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📌 Takeaways

It’s good to experiment with “obvious cases that would never happen in real life” to see how a programming language actually handles them. And also because it's fun.

Here are a few more I found interesting:

isNaN(null)        // false; null is coerced to 0
isNaN([])          // false; [] coerced to 0
isNaN("")          // false; "" coerced to 0
isNaN(true)        // false; coerced to 1
isNaN(false)       // false; coerced to 0
isNaN("text")      // true
isNaN({})          // true; object can't be coerced to a number
isNaN(undefined)   // true; undefined, not be coerced to a number; NaN

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Got a favorite weird JavaScript case? Share it in the comments. Always open to learning something new.

Comments

[Nguyễn Thanh Tuấn]

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