The Sneaky JavaScript Bug That Hides in Plain Sight — A 0 That Broke My Code

👋 Intro

We’ve all written that handy little twoSum function — loop through an array, find two numbers that add up to a target, return their indices. Easy, right?

Well… almost.

Recently, while working on a real project, I stumbled into one of JavaScript’s classic gotchas — and it all came down to a single number: 0.

Let’s break down the bug, the fix, and how you can avoid it in your own code.

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🧠 The Setup: Simple Two Sum

Here's a basic implementation many devs (me included!) start with:

const twoSum = (arr, target)=>{
    const map = {};
    for(let i = 0; i < arr.length; i++){
        const complement = target - arr[i];
        if(map[complement]){
            return [map[complement], i];
        }
        map[arr[i]] = i;
    }
    return [];
}

This works fine in many cases:

console.log(twoSum([2, 7, 11, 15], 9)); // [0, 1]

✅ Perfect.

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🐞 Then Comes the Bug

Now try this:

console.log(twoSum([3, 2, 4], 6)); // 🤔

Expected: [1, 2]
Actual: []

Wait… what?

Here’s what’s happening:

• We store indices in map.
• When complement = 3, map[3] = 0
• But then we do: if (map[complement])
• Since map[complement] = 0, and 0 is falsy in JavaScript, the condition fails.
• The code skips the match, even though it's correct.

Ouch.

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🧯 The Fix: Use hasOwnProperty

Instead of checking if map[complement] is truthy, check if the key exists:

if (map.hasOwnProperty(complement)) {

✅ Final Working Version:

const twoSum = (arr, target)=>{
    const map = {};
    for(let i = 0; i < arr.length; i++){
        const complement = target - arr[i];
        if(map.hasOwnProperty(complement)){
            return [map[complement], i];
        }
        map[arr[i]] = i;
    }
    return [];
}

Now this works:

console.log(twoSum([3, 2, 4], 6)); // [1, 2]

Boom 💥

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💡 Takeaway

In JavaScript, 0, null, undefined, false, '', and NaN are all falsy.
So using truthy checks (if (obj[key])) can break your code if the value is legitimately 0 or false.

Always use:

• if (obj.hasOwnProperty(key)) — ✅ safe
• Or: if (key in obj) — also works
• Or even: typeof obj[key] !== 'undefined'

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🛠 Real-World Use Case

I hit this bug in a small front-end app where I was mapping component state to form field IDs. The field with ID 0 kept getting skipped on validation. Yep — same problem. 😅

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📢 TL;DR

• Avoid if(obj[key]) if key can map to 0, false, or other falsy values.
• Use hasOwnProperty() or a safer pattern.
• JavaScript truthiness is a powerful feature — but can sneak up on you.