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TypeScript: Types, Interfaces & Generics

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Core TypeScript type system: types, interfaces, and generics.

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Three primitive types in TypeScript

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1Three primitive types in TypeScript
string, number, and boolean
2Why should you use lowercase type names (string, number, boolean)?
The capitalized versions (String, Number, Boolean) refer to special built-in types that rarely appear in your code
3Specify the type of an array in TypeScript
Use number[] or Array<number> syntax (both are equivalent)
4'any' type in TypeScript
A special type that disables all type checking - you can access any properties, call it as a function, or assign it to any type
5What compiler flag prevents implicit 'any' types?
noImplicitAny
6Where do type annotations go in TypeScript?
After the thing being typed (not before like int x = 0 in other languages)
7Type inference
TypeScript automatically determining types based on context, like inferring a variable's type from its initializer
8Add a type annotation to a function parameter
After the parameter name: function greet(name: string) { ... }
9Specify a function's return type
After the parameter list: function getFavoriteNumber(): number { ... }
10Contextual typing
When TypeScript infers parameter types based on how and where a function is used (e.g., in array.forEach)
11Define an object type inline
List properties and types: function printCoord(pt: { x: number; y: number }) { ... }
12Make an object property optional
Add a ? after the property name: { first: string; last?: string }
13What happens when you access an optional property?
You get undefined if it doesn't exist, so you must check for undefined before using it
14Union type
A type formed from two or more other types, representing values that may be any one of those types
15Define a union type
Use the pipe operator: number | string
16Narrowing
When TypeScript deduces a more specific type for a value based on the structure of the code (e.g., after a typeof check)
17How can you narrow a union type?
Use typeof checks, Array.isArray(), or other runtime checks to determine which type it is
18Type alias
A name for any type, defined with: type Point = { x: number; y: number }
19Can type aliases be used for union types?
Yes: type ID = number | string
20Interface in TypeScript
A way to name an object type with its properties and their types
21Main difference between interfaces and type aliases
Interfaces can be re-opened to add new properties (declaration merging), type aliases cannot
22Extend an interface
interface Bear extends Animal { honey: boolean }
23Extend a type alias
Use intersection: type Bear = Animal & { honey: boolean }
24Can you reopen a type alias to add properties?
No, a type cannot be changed after being created
25Type assertion
A way to tell TypeScript a more specific type than it knows: document.getElementById('canvas') as HTMLCanvasElement
26Alternative syntax for type assertions
Angle brackets: <HTMLCanvasElement>document.getElementById('canvas') (not usable in .tsx files)
27Do type assertions affect runtime behavior?
No, they are removed by the compiler and have no runtime impact
28Literal type
A type that refers to a specific string or number value, like 'hello' or 42
29How are const variables typed vs let variables?
const gets a literal type (const x = 'hello' → type 'hello'), let gets a general type (let x = 'hello' → type string)
30How can you use literal types with unions?
To restrict values to a specific set: alignment: 'left' | 'right' | 'center'