TSConfig

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:

ts
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;
          
(property) GameSettings.speed: "fast" | "medium" | "slow"
settings.quality;
           
(property) GameSettings.quality: "high" | "low"
 
// Unknown key accessors are allowed on
// this object, and are `string`
settings.username;
            
(index) GameSettings[string]: stringTry

Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.

ts
const settings = getSettings();
settings.speed;
settings.quality;
 
// This would need to be settings["username"];
settings.username;
Property 'username' comes from an index signature, so it must be accessed with ['username'].4111Property 'username' comes from an index signature, so it must be accessed with ['username'].
            
(index) GameSettings[string]: stringTry

The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists.