7 Subtle JavaScript Behaviors That Can Trip You Up (And How to Avoid Them

## 🧠 Introduction

JavaScript is one of the most powerful—and sometimes quirky—languages in modern development. Whether you're a beginner or a seasoned dev, these subtle behaviors can cause unexpected bugs.

Let’s dive into 7 common “gotchas” and how to handle them like a pro.

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1️⃣ NaN is Not Equal to Itself

console.log(NaN === NaN); // false

😲 Yes, NaN is the only value in JavaScript that is not equal to itself.

✅ Use Number.isNaN(value) instead:

Number.isNaN(NaN); // true

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2️⃣ typeof null is 'object'

console.log(typeof null); // 'object'

🧠 This is a long-standing bug in JavaScript and won’t be fixed due to backward compatibility.

✅ Always explicitly check for null:

if (value === null) {
  // handle null
}

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3️⃣ [] + [] Returns an Empty String

console.log([] + []); // ''

Because arrays are coerced to strings, and [] becomes '', the result is an empty string. Wild, right?

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4️⃣ [] == ![] is true

console.log([] == ![]); // true

👉 Why? JavaScript performs weird coercions here. ![] becomes false, and [] == false becomes true due to type coercion.

✅ Use strict equality (===) to avoid this.

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5️⃣ Function Declarations Inside Blocks (❌ ES5 Pitfall)

if (true) {
  function sayHi() {
    console.log("hi");
  }
}

sayHi(); // Throws in strict mode

✅ Avoid declaring functions inside blocks. Use function expressions instead:

if (true) {
  const sayHi = () => console.log("hi");
}

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6️⃣ Implicit global Variables

function foo() {
  bar = 5; // No 'let', 'const', or 'var'
}

foo();
console.log(bar); // 5 (attached to global object 😱)

✅ Always declare variables with let, const, or var.

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7️⃣ Objects as Keys Are Converted to Strings

const obj = {};
const key = {};

obj[key] = "value";

console.log(obj); // { "[object Object]": "value" }

✅ Use Map when you want objects as keys:

const map = new Map();
map.set({}, "value");

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✨ Final Thoughts

JavaScript is beautiful—but it has quirks. Understanding these helps you avoid subtle bugs and write cleaner, more predictable code.

💬 Found this helpful? Leave a comment or share your favorite JS gotcha!