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Common errors

The type errors you meet first (TS2304, TS2322, TS18048, TS7006, TS2339, TS2451) and run-time errors.

Runs in Zerethon IDE · TypeScript 7 Official TypeScript documentation ↗
  • TS2304: Cannot find name 'x' — a typo, or a variable declared elsewhere (or a missing import).
  • TS2322: Type 'string' is not assignable to type 'number' — the value does not match the declared type; convert it (Number(text)) or fix the type.
  • TS2345: Argument of type … is not assignable to parameter of type … — the same for a function argument.
  • TS18048: 'x' is possibly 'undefined' — check first: if (x !== undefined), x ?? fallback, or x?.name.
  • TS2451: Cannot redeclare block-scoped variable 'name' — a file without import / export is a script, and its top-level names are global: they clash with the same name in another script file, or with a built-in global such as name. Rename the variable, or add export {}; to make the file a module.
  • TS7006: Parameter 'x' implicitly has an 'any' type — add a type to the parameter.
  • TS2339: Property 'name' does not exist on type … — a typo, or the type does not declare that field.
  • Run-time errors (TypeError, RangeError, …) are JavaScript errors; the stack trace names the .ts file and line.
main.ts 
function parsePort(text: string): number | undefined {
  const port = Number(text);
  return Number.isInteger(port) ? port : undefined;
}

const port = parsePort('80x');
// port + 1 would be TS18048: 'port' is possibly 'undefined'.
console.log(port === undefined ? 'invalid port' : port + 1);
console.log(parsePort('8080') ?? 'invalid port');

Output

invalid port
8080

Related topics

Official references