Question 1

What is control-flow narrowing?

Accepted Answer

TS refining types based on checks like typeof, equality, and returns. Enables safe property access after guards. Understand what resets narrowing.

Question 2

How does typeof narrowing work and its limits?

Accepted Answer

Distinguishes primitives; typeof null is object historically — handle null separately. Doesn't narrow custom class instances well — use instanceof.

Question 3

Discriminated unions — pattern?

Accepted Answer

Shared literal tag field to distinguish variants for exhaustive switches. Preferred over boolean flags soup. Enables never checks for exhaustiveness.

Question 4

Truthy narrowing pitfalls?

Accepted Answer

Empty string/0/NaN are falsy — may incorrectly narrow away valid values. Prefer explicit null checks for those domains. == null catches null and undefined.

Question 5

in operator narrowing?

Accepted Answer

Checks property existence to narrow unions of objects. Careful with optional properties and prototypes. Prefer discriminants when possible.

Question 6

Assertion functions (asserts)?

Accepted Answer

Functions that throw and tell TS a condition is true afterward. Useful for shared assertDefined helpers. Must actually throw on failure.

Question 7

Why does narrowing fail after assigning to a variable?

Accepted Answer

Aliasing can invalidate narrowing if TS can't prove the alias is unchanged. Inline checks or use const + careful patterns. Known footgun with property stores.

Type Narrowing TypeScript Interview Questions

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