Papliba

JavaScript Gotchas

"" == 0 and 1 < x < 3

TL;DR

• Use ===, not ==.
• Don’t chain comparisons. Use x > 1 && x < 3.

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Why "" == 0 is true

• == coerces types.
• "" becomes 0, so it’s 0 == 0 → true.

"" == 0          // true  (coercion)
"" === 0         // false (no coercion)
Number("") === 0 // true  (explicit)

Rule: Prefer === / !==.

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Why 1 < x < 3 is broken

• JS doesn’t chain comparisons like math.
• It evaluates left-to-right:
  1. 1 < x → true or false
  2. That boolean becomes a number (true → 1, false → 0)
  3. 1 < 3 or 0 < 3 → always true

let x = 100;
1 < x < 3   // (true) < 3 → 1 < 3 → true

x = -5;
1 < x < 3   // (false) < 3 → 0 < 3 → true

Correct:

x > 1 && x < 3

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Quick Rules

• Equality: use === / !==.
• Ranges: lower < x && x < upper.
• Be explicit with conversions:

  const n = Number(s);
  

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ESLint (recommended)

Enable rules to catch these issues.

{
  "rules": {
    "eqeqeq": ["error", "always"],
    "no-implicit-coercion": "error"
  }
}