// Object literal may only specify known properties, and 'd' does not exist in type 'FromIndex'. However, if you are modeling existing JavaScript you can get around it with an intersection type. Let's start with something big! to capture vocabulary types, described on the next page. However, if you are modeling existing JavaScript you can get around it with an intersection type. to allow typing stuff like: API consideration when adding index signatures. index in { [index:string] : {message: string} } has no significance for TypeScript and is only for readability. { [count: number] : SomeOtherTypeYouWantToStoreEgRebate }. But let's not go there just yet. As soon as you have a string index signature, all explicit members must also conform to that index signature. This is often used in JavaScript to access properties of an object. But let's not go there just yet. This is shown below: This is to provide safety so that any string access gives the same result: An index signature can require that index strings be members of a union of literal strings by using Mapped Types e.g. The following shows an example of the error you will encounter without using an intersection: Here is the workaround using an intersection type: Note that even though you can declare it to model existing JavaScript, you cannot create such an object using TypeScript: in JavaScript (and hence TypeScript) can be accessed with a, to hold a reference to any other JavaScript, . The specification of the vocabulary can be deferred generically: This is not a common use case, but TypeScript compiler supports it nonetheless. These signatures are a way to signal to the type system that users can access arbitrarily-named properties. If you pass any other object to the index signature the JavaScript runtime actually calls .toString on it before getting the result. Baby steps. Accordingly, if we have both a string index signature and a number index signature, the property type of the former must be a supertype of the latter. index signature in typescript; index.js:1 Warning: Failed prop type: The prop `expandableRows` is marked as required in `<>` indexable type in ts; init empty object typescript; initialize empty array typescript; injection of generic services in angular; inline scripts encapsulated in