Literal types and value narrowing in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a literal type in TypeScript?

A literal type is a type that represents a specific fixed value, like a string literal "hello" or a number literal 42, instead of a general type like string or number.

beginner

How does TypeScript narrow types using literal types?

TypeScript narrows types by checking values against literal types in conditions, so inside an if statement checking for a specific literal, the type becomes that literal type.

beginner

What will be the type of variable 'x' after this check?<br>

if (x === "open") { /* here */ }

Inside the if block, 'x' is narrowed to the literal type "open" because the condition ensures 'x' equals that exact string.

intermediate

Why are literal types useful in function parameters?

Literal types let you restrict function parameters to specific allowed values, helping catch errors early and making code easier to understand.

intermediate

Explain how union types and literal types work together for narrowing.

When a variable has a union of literal types, TypeScript can narrow it to one specific literal inside conditional checks, allowing precise control over possible values.

What type does TypeScript assign to a variable declared as const status = "success";?

A unknown

B string

C any

D "success" (literal type)

Because 'status' is a const assigned to a string literal, TypeScript infers the literal type "success".

Given let mode: "auto" | "manual";, what happens inside if (mode === "auto") block?

A mode is narrowed to "auto"

B mode remains "auto" | "manual"

C mode is any

D mode is string

The condition narrows 'mode' to the literal type "auto" inside the if block.

Which of these is NOT a literal type in TypeScript?

A "hello"

B 42

C boolean

D true

"boolean" is a general type, not a literal type. Literal types are specific values like true or false.

How does TypeScript treat a variable declared as let x = 10; without a type annotation?

A x has type number

B x has type 10 (literal)

C x has type any

D x has type unknown

Variables declared with let and assigned a number get the general number type, not the literal type.

What is the benefit of using literal types in TypeScript?

A They convert types to any

B They allow precise control over allowed values

C They disable type checking

D They make variables mutable

Literal types restrict variables to exact values, improving safety and clarity.

Describe what literal types are and how TypeScript uses them to narrow values in code.

Think about how checking a variable against a specific value changes its type.

Explain how union types combined with literal types help in writing safer TypeScript functions.

Consider how limiting inputs to specific values prevents errors.