Type System Basics

Preview — 3 of 10 questions

What is the correct way to annotate a variable in TypeScript?

javascript

// Which is valid?
let name = "Alice";           // A
let name: string = "Alice";   // B
let name<string> = "Alice";   // C
let name::string = "Alice";   // D

Alet name = "Alice" — no annotation needed

Blet name: string = "Alice" — colon syntax

Clet name<string> = "Alice" — angle bracket syntax

Dlet name::string = "Alice" — double colon syntax

What type does TypeScript infer for score?

javascript

let score = 42;
score = "high"; // ?

Aany — TypeScript can't infer from a number literal

Bnumber — TypeScript infers number from the initializer, so assigning a string is an error

Cnumber | string — TypeScript infers a union because both values are assigned

D42 — TypeScript infers the literal type

Which two syntaxes are equivalent for typing an array of strings?

javascript

// Option 1
let tags: string[];

// Option 2
let tags: Array<string>;

AThey are different — string[] is read-only, Array<string> is mutable

Bstring[] only works with primitives; Array<string> works with all types

CArray<string> allows null elements; string[] does not

DThey are equivalent — both declare a mutable array of strings

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