JavaScript Problem: Why Does This Return the Wrong Result?

programming
1

I came across a small JavaScript bug that looks correct at first glance.

function isAdult(age) {
  if (age = 18) {
    return true;
  }

  return false;
}

console.log(isAdult(16));

The code prints true, even though the age is 16.

What exactly is happening here?

And would you use == or === to fix it?

I’m curious how others would explain this bug to someone who is just starting with JavaScript. Is there an easy way to remember the difference between = and ===?

2

The issue is = means assignment, so age = 18 changes age to 18. Since 18 is truthy, the condition returns true.

Use === for comparison:

if (age === 18)

Easy reminder: = assigns, === compares strictly.

3

I like that reminder. It’s like when I’m drawing, I use a light pencil for a guide line, but a dark pen for the final shape. Two different tools for different jobs.

4

That’s a great explanation of the assignment vs. comparison difference.

5

You’re right, that’s exactly what’s happening. The single equals sign assigns the value, making the condition always truthy.

For comparison, you’ll definitely want to use ===. It checks both value and type, which is usually what you need in JavaScript.

6

That’s a good observation about the assignment operator. I’ve seen that exact mistake lead to some truly baffling behavior in larger systems.

7

It’s so easy to make that mistake. I remember one time I spent hours debugging a CSS issue that was just a missing semicolon. :woman_facepalming:

8

That’s the kind of thing that makes you question your career choices at 3 AM.

I’ve seen similar issues with content migrations where a small character error in a template can cascade into hundreds of broken pages. It’s a reminder that precision in syntax is critical, even for seemingly minor details.