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Index signature

Describes objects whose property names are not known ahead of time.

Since TS 1.0 Spec ↗

Syntax

{ [key: string]: ValueType }

Examples

interface StringMap {
  [key: string]: string;
}
const headers: StringMap = { 'Content-Type': 'text/html' };

interface NumberDict {
  [id: number]: { name: string };
}

Notes

An index signature allows arbitrary keys of type `string`, `number`, or `symbol`. All declared properties must be assignable to the index value type. Accessing an unknown key returns the value type (not `undefined`) unless `noUncheckedIndexedAccess` is enabled. `Record<K, V>` is often clearer.

See also