Type Coercion
mediumComplete guide with examples and interview questions · 25 min
Say it out loud in an interview — clear, simple, no textbook wording.
If someone asks what type coercion is, I'd say: JavaScript converts values between types automatically when an operator needs it to. You never called Number() — the operator did it on your behalf.
The mental model that makes the weird stuff predictable: the operator decides the target type. + says "string if I see one, otherwise numbers." -, *, /, and < say "numbers, always." if, &&, and ! say "booleans." Once you know what the operator wants, the result stops being magic.
Interviewers probe the two places this bites: objects turning into primitives (valueOf/toString, hint order) and the falsy list. Nail those and every "trick question" is just arithmetic.
Key takeaways
- •Coercion is operator-driven: the operator picks a target type, then converts the operands
- •`+` concatenates if either side (after ToPrimitive) is a string; every other arithmetic operator converts to numbers
- •Objects become primitives via Symbol.toPrimitive first, then valueOf/toString in hint order — "string" hint tries toString first
- •Falsy is exactly 0, -0, 0n, "", null, undefined, NaN, false — everything else, including [] and {}, is truthy
- •Chained comparisons don't chain: 1 < 2 < 3 evaluates left to right as (1 < 2) < 3
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