tapester tapester - 1 year ago 228
TypeScript Question

What does this type declaration mean in Typescript?

I'm new to Typescript and I'm reading over someone else's code and having trouble with this declaration:

private somevar: { [s: string]: string };

What type is this? Square brackets indicate an array, but I'm confused about exactly what shape this would be in.

Answer Source

It's an indexable type. Looking at this variable definition's type expression:

let myIndexVar: { [key: string]: number; };
  • : { ... } means it's an object.
  • [key: string]: number; is the index signature of the object.

Within the index signature:

  • [key: string] defines the name of the key—key—and the type of the key—string.
  • : number; is the type of the value—number.

These types are used like so:

let myIndexVar: { [key: string]: number; } = {};

myIndexVar["key"] = 4; // key is string, value is number

Note that you can give the key any name you like. Giving it a descriptive name is helpful to tell what the key is, though the variable name should also do that:

Intellisense example

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