Index Signatures · TypeScript Deep Dive

A tip on adding #index signatures to your #JavaScript : 

#TypeScript #ReactJS #AngularJS 🌹

  • An in JavaScript (and hence TypeScript) can be accessed with a string to hold a reference to any other JavaScript object.
  • Remember we said it can store any JavaScript object, so lets store a class instance just to show the concept:

    Also remember that we said that it can be accessed with a string.

  • If you pass some any other object to the index signature the JavaScript runtime actually calls on it before getting the result.
  • First off, because JavaScript implicitly calls on any object index signature, TypeScript will give you an error to prevent beginners from shooting themselves in the foot (I see users shooting themselves in their feet when using JavaScript all the time on stackoverflow):

    The reason for forcing the user to be explicit is because the default implementation on an object is pretty awful, e.g. on v8 it always returns :

    Of course is supported because

    Quick note: are also valid and supported by TypeScript.

  • 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 e.g.:

    This is often used together with to capture vocabulary types, described on the next page.

An Object in JavaScript (and hence TypeScript) can be accessed with a string to hold a reference to any other JavaScript object.

@basarat: A tip on adding #index signatures to your #JavaScript :

#TypeScript #ReactJS #AngularJS 🌹

in JavaScript (and hence TypeScript) can be accessed with a string to hold a reference to any other JavaScript object.

Here is a quick example:

. Remember we said it can store any JavaScript object, so lets store a class instance just to show the concept:

on it before getting the result. This is demonstrated below:

is used in an index position.

). Here is a simple array example:

So that’s JavaScript. Now let’s look at TypeScript graceful handling of this concept.

on any object index signature, TypeScript will give you an error to prevent beginners from shooting themselves in the foot (I see users shooting themselves in their feet when using JavaScript all the time on stackoverflow):

are also valid and supported by TypeScript. But let’s not go there just yet. Baby steps.

index signature, all explicit members must also conform to that index signature. This is shown below:

An index signature can require that index strings be members of a union of literal strings e.g.:

to capture vocabulary types, described on the next page.

This is not a common use case, but TypeScript compiler supports it nonetheless.

Index Signatures · TypeScript Deep Dive

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