website·Indexed 29 days ago
https://rayadaschn.github.io/effective-typeScript-2nd-edition-zhA comprehensive guide to TypeScript best practices, featuring items on advanced type modeling (Template Literal Types, Optional Never Properties, Brands), compiler performance, and migration strategies from JavaScript. Covers topics such as exhaustiveness checking with never types, TSDoc for API comments, and the relationship between type checking and unit testing.
unknown is a type-safe alternative to any. Use unknown when you know a value exists but do not know or care about its specific type. Unlike any, which disables type checking by allowing any operation and assignment, unknown is a 'top type'—any type can be assigned to it, but it cannot be assigned to other types (except any or another unknown) without explicit type narrowing or assertions.When managing dependencies in TypeScript, you must ensure synchronization between three distinct versions to avoid type errors or runtime crashes:
react@18.2.0).@types/react@18.2.23). The major and minor versions should match the package, while the patch version reflects fixes to the types themselves.Failure to sync these can lead to scenarios where code passes type checking but fails at runtime (types are newer than the library) or fails type checking despite the library supporting the feature (library is newer than the types).
tsc), which affects CI and build artifact generation, and editor latency (tsserver), which affects the responsiveness of the IDE (e.g., delay in error reporting). Optimizations should be targeted based on which of these is the bottleneck.Follow these guidelines to maintain type safety in TypeScript:
: Type) over type assertions (as Type).TypeScript symbols exist in two distinct spaces: Type Space and Value Space. Symbols in the type space are used for static type checking and are erased during compilation to JavaScript. Symbols in the value space exist at runtime.
Key distinctions:
type and interface. These are erased at runtime.const, let, var, or function implementations. These persist in the compiled JavaScript.class and enum introduce both a type (based on the structure) and a value (the constructor function or the enum object).Context determines which space a symbol belongs to:
:) or the as keyword are in the type space.=) are in the value space.interface Person { // Type Space
first: string
last: string
}
const jane: Person = { first: 'Jane', last: 'Jacobs' }
// ^ Value Space ^ Type Space ^ Value Spacetsc) performs two independent tasks: transpiling TypeScript/JavaScript to an older version of JavaScript and checking for type errors. Because these processes are decoupled, TypeScript types are 'erased' during compilation and do not exist at runtime. Consequently, type annotations do not affect the runtime behavior or performance of the generated JavaScript code. Notably, code with type errors can still be compiled into executable JavaScript output.