Basic Interview Questions

TypeScript is a statically typed superset of JavaScript made by Microsoft. It adds type annotations and a compiler (tsc) that checks types and emits plain JavaScript. Benefits: errors are caught at compile time instead of in production, better autocomplete and refactoring, and self-documenting code. Types are erased, so there is no runtime cost and no runtime type checking either.

Neither browsers nor Node.js run TypeScript types. tsc (or esbuild/SWC/Babel) transpiles it to JavaScript, which is then run. Node.js 22.6+ can strip simple type annotations itself (--experimental-strip-types), and tools like tsx do it on the fly, but none of these check types; only tsc does.

Use unknown for untrusted data like JSON.parse results and catch variables.

TypeScript infers types from values: let n = 5 is number, a function's return type is inferred from its return statements. Annotate function parameters (always required in strict mode), variables declared without a value, empty arrays, and usually the return types of exported functions. Let inference handle local variables.

"strict": true enables a group of checks: strictNullChecks, noImplicitAny, strictFunctionTypes, strictPropertyInitialization, strictBindCallApply, useUnknownInCatchVariables, noImplicitThis and alwaysStrict. The most important is strictNullChecks, which stops null/undefined from being assigned to every type.

function len(s: string | null): number {
  return s.length; // strictNullChecks: 's' is possibly 'null'
}

Line 2: error TS18047: 's' is possibly 'null'.

void means "the return value should not be used". A function type returning void can be implemented by a function that returns something (e.g. arr.forEach(x => list.push(x)) is allowed even though push returns a number). undefined means the function must literally return undefined.