| display | No Property Access From Index Signature |
|---|---|
| oneline | Enforces using indexed accessors for keys declared using an indexed type. |
This setting ensures consistency between accessing a field via the "dot" (obj.key) syntax, and "indexed" (obj["key"]) and the way which the property is declared in the type.
Without this flag, TypeScript will allow you to use the dot syntax to access fields which are not defined:
// @errors: 4111
declare function getSettings(): GameSettings;
// ---cut---
interface GameSettings {
// Known up-front properties
speed: "fast" | "medium" | "slow";
quality: "high" | "low";
// Assume anything unknown to the interface
// is a string.
[key: string]: string;
}
const settings = getSettings();
settings.speed;
// ^?
settings.quality;
// ^?
// Unknown key accessors are allowed on
// this object, and are `string`
settings.username;
// ^?Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.
// @errors: 4111
// @noPropertyAccessFromIndexSignature
declare function getSettings(): GameSettings;
interface GameSettings {
speed: "fast" | "medium" | "slow";
quality: "high" | "low";
[key: string]: string;
}
// ---cut---
const settings = getSettings();
settings.speed;
settings.quality;
// This would need to be settings["username"];
settings.username;
// ^?The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists.