Equality narrowing in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is equality narrowing in TypeScript?

Equality narrowing is a feature where TypeScript uses equality checks (like === or !==) to narrow down the possible types of a variable within a code block.

beginner

How does TypeScript narrow types after an if (x === null) check?

Inside the if block, TypeScript knows x is exactly null. Outside, it knows x is not null and narrows the type accordingly.

beginner

Given <code>let value: string | number;</code> what happens after <code>if (typeof value === 'string')</code>?

Inside the if block, TypeScript narrows value to string. Outside, it narrows value to number.

intermediate

Can equality narrowing work with custom types or only primitives?

Equality narrowing works with any types where equality checks make sense, including primitives like string, number, null, and also with literal types and enums.

intermediate

What happens if you compare two variables of union types with === in TypeScript?

TypeScript narrows the types of both variables to the overlapping types that can be equal, helping catch impossible comparisons or refine types inside the check.

What does TypeScript do after if (x === null)?

A Narrows <code>x</code> to <code>undefined</code> inside the block

B Narrows <code>x</code> to <code>null</code> inside the block

C Narrows <code>x</code> to <code>string</code> inside the block

D Does not narrow <code>x</code>

Inside the if block, TypeScript knows x must be null.

Given let val: string | number; what is val inside if (typeof val === 'string')?

A string

B boolean

C string | number

D number

The typeof check narrows val to string inside the block.

Does equality narrowing work with enum types in TypeScript?

A Only with numeric enums

B No, enums are not narrowed

C Only with string enums

D Yes, it narrows based on enum values

Equality narrowing works with enums by narrowing to specific enum values.

What happens if you compare two union type variables with ===?

A TypeScript narrows both to overlapping types

B TypeScript throws an error

C TypeScript ignores the check

D TypeScript widens the types

TypeScript narrows both variables to the types that can be equal.

Which operator triggers equality narrowing in TypeScript?

A +

B -

C ===

D *

The strict equality operator === triggers equality narrowing.

Explain how equality narrowing helps TypeScript understand variable types inside an if statement.

Think about how checking if a variable equals a value changes what TypeScript knows about it.

Describe what happens when you compare two union type variables with === in TypeScript.

Consider how TypeScript finds common types that can be equal.