Which JavaScript equality and coercion rules matter in application code, and how do you avoid surprising comparisons?

Prioritizes explicit normalization, strict equality, object identity, nullish handling, and NaN over coercion puzzles.

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TL;DR

I use strict equality by default because it compares values without converting between types. Objects compare by identity, so two separate objects with the same fields are not equal. I normalize external strings, numbers, and booleans at the boundary instead of relying on implicit coercion throughout the application. I also treat nullish values and NaN deliberately.

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Practice focus

Whether the candidate knows the small set of comparison rules that prevent real application defects.

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Answer path

Core answer first, then checkpoints toward a stronger answer.

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Core answer
Start here. This is the direct spoken answer to practice first.
Overview
The interview value is safe application behavior, not predicting deliberately obscure coercion puzzles.
I use strict equality by default because it compares values without converting between types. Objects compare by identity, so two separate objects with the same fields are not equal. I normalize external strings, numbers, and booleans at the boundary instead of relying on implicit coercion throughout the application. I also treat nullish values and NaN deliberately.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
A form input is a string even when it looks numeric, so I parse and validate it before comparing it with a numeric identifier. I use value == null only if the codebase deliberately wants the narrow loose-equality behavior that matches both and ; otherwise I write the cases explicitly. For , is clearer than equality because is not equal to itself.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Using loose equality as a shortcut for mixed input types.
Expecting separate object literals with the same fields to be equal.
Comparing NaN with ordinary equality.
Turning the answer into a list of obscure coercion puzzles.

Which JavaScript equality and coercion rules matter in application code, and how do you avoid surprising comparisons?

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