Lesson 12 of 80

Equality & Type Coercion

Q: Explain the difference between == and ===, and walk through why [] == false is true.

A strong answer covers: === compares type and value with no conversion; == converts operands to a common type first, following the Abstract Equality Comparison algorithm; For [] == false: the object [] is converted to a primitive (toString -> ""), then both sides are coerced numerically ("" -> 0, false -> 0), giving 0 == 0 -> true; null and undefined are special-cased: == to each other only, not to 0, "", or false.

== converts types before comparing; === refuses to. Knowing the handful of coercion rules turns 'JS is weird' into 'JS is predictable, if you know the rule.'

12 min read

Check your understanding

Every option explains itself — why the right one is right, and why each wrong one is wrong. Miss one and it comes back later on a spaced-repetition schedule.

Question 1 of 2

What does `null == undefined` evaluate to, and why?

true — null and undefined are hard-coded as equal to only each othertrue — both coerce to 0 firstfalse — they're different types so == can't match them

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Interview questions on Equality & Type Coercion

What interviewers actually ask about this, and what a strong answer has to cover.

Explain the difference between == and ===, and walk through why [] == false is true.

A strong answer covers

=== compares type and value with no conversion; == converts operands to a common type first, following the Abstract Equality Comparison algorithm
For [] == false: the object [] is converted to a primitive (toString -> ""), then both sides are coerced numerically ("" -> 0, false -> 0), giving 0 == 0 -> true
null and undefined are special-cased: == to each other only, not to 0, "", or false

Likely follow-ups

Is NaN === NaN? What about Object.is(NaN, NaN)? Why the difference?
Why is the industry convention to default to === and only reach for == in specific cases like `x == null`?
Give an example where == coercion is genuinely useful rather than a footgun.

Practice these with the AI interviewer →

The plain-language idea

=== compares value and type — no conversion, ever. == first tries to make both sides the same type, then compares. The famous "weirdness" ([] == false, "" == 0) isn't random — it's a small, fixed set of coercion rules being followed exactly.

"" == 0        // true  — "" coerces to 0
[] == false    // true  — [] -> "" -> 0, false -> 0
null == undefined  // true  — special-cased, equal only to each other
null === undefined // false — different types, no coercion

0 === -0       // true
NaN === NaN    // false — NaN is never equal to anything, including itself

The one rule worth memorizing

When == compares a primitive to an object (like [] == false), the object is converted to a primitive first (via toString/valueOf), then the usual numeric coercion rules apply to both sides. null and undefined are the one hard-coded exception: they're == to each other and to nothing else — not even 0 or "".

[] == false: coercion, step by stepStep 1 of 4

[] == false — different types. == tries to coerce them to a common type before comparing.

What people usually miss

Watch out

== isn't "wrong," it's a specific algorithm. The Abstract Equality Comparison in the spec is a real, fixed table of rules — the reputation for chaos comes from not knowing the table, not from actual randomness.
The industry default is === everywhere, with == null as the one common intentional exception (it catches both null and undefined in one check).
Falsy values aren't all "equal" to each other. 0 == "" is true, but 0 == "0" is also true while "" == "0" is false — coercion is pairwise, not transitive.

Why this matters in interviews

"What's the difference between == and ===, and can you name three surprising == results?" checks whether you actually understand coercion rules or just parrot "always use triple equals" without knowing why.