TypeScript cheat sheet

1. 1Types and the compilerWhat TypeScript is, what the compiler does with your code and the handful of types everything else is built from.7 concepts

   1. 1Type checking and type erasureTypeScript is JavaScript plus a static type system.
   2. 2Primitive and literal typesstring, number, boolean, bigint, symbol, null and undefined are the primitive types.
   3. 3Arrays, tuples and readonly arraysstring[] (or Array<string>) is a list of any length.
   4. 4Object types and optional propertiesAn object type lists property names and their types.
   5. 5any and unknownany switches type checking off for a value.
   6. 6never and voidvoid means a function returns nothing useful.
   7. 7Strict null checksWith strictNullChecks (part of strict), null and undefined are separate types that must be listed explicitly, as in string | null, and handled before…

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2. 2Inference and assignabilityHow TypeScript works out types you did not write, and the rules that decide whether one type fits where another is expected.7 concepts

   1. 1Type inferenceThe compiler infers types from values: const n = 3 is 3, let n = 3 is number, and a function's return type comes from what it returns.
   2. 2Structural typingTypeScript compares types by shape, not by name.
   3. 3Excess property checksWhen you pass an object literal straight to a typed slot, extra properties the type does not declare are an error.
   4. 4Widening and as constMutable bindings widen literals: let m = 'GET' is string.
   5. 5The satisfies operatorexpr satisfies T checks that a value fits T without changing the value's inferred type.
   6. 6Type assertions and non-null !value as T tells the compiler to treat a value as T; value! says it is not null or undefined.
   7. 7Contextual typingWhen a function or literal is written where a type is already expected, such as a callback argument, TypeScript infers its parameter types from that context.

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3. Unions and narrowingUnions describe values that can be one of several types; narrowing is how the compiler follows your checks to know which one you have.7 conceptsOpens with credits
4. GenericsGenerics are type parameters: they let one function or type work over many types while keeping the link between input and output.6 conceptsOpens with credits
5. Built-in utility typesGeneric types shipped with TypeScript that transform other types. Learn these before writing your own type-level code.6 conceptsOpens with credits
6. Type-level toolsThe operators behind the utility types. They let you compute new types from existing ones, so one source of truth drives the rest.8 conceptsOpens with credits
7. Interfaces, classes and enumsHow TypeScript describes object-oriented code, and the few features that are TypeScript syntax with a runtime cost.6 conceptsOpens with credits
8. Modules, tsconfig and strictnessHow TypeScript files import each other, how the compiler is configured and which flags are worth turning on in every project.7 conceptsOpens with credits
9. JavaScript libraries and runtime dataWhere the type system meets code and data it cannot see: untyped packages, declaration files and values that arrive at runtime.6 conceptsOpens with credits
10. 10Glossary, A to ZEvery concept in the sheet and the smaller terms it uses, in one list.88 terms

Types, inference, narrowing, generics, type-level tools and strict configuration for real TypeScript codebases.