Why square brackets in the type declaration of an object key?

I need this:

Impact: = 0: "Minor", 1: "Major", 2: "Critical" ;


If I want to annotate the type explicitly, it is:

Impact: [key: number]: string = 0: "Minor", 1: "Major", 2: "Critical" ;


Why not:

Impact: number: string = 0: "Minor", 1: "Major", 2: "Critical" ;


What do the square brackets mean? Is key a reserved word?

key

1 Answer
1

The square brackets are mnemonic for the square brackets that you would use to access an element:

let Impact: [key: number]: string = ...;
console.log(Impact[0]);


Compare to a call signature, which can be declared like this:

let Impact: (key: number): string = ...;
// equivalent shorthand:
// let Impact: (key: number) => string;
console.log(Impact(5));


The name key is an arbitrary parameter name in both cases.

key

The number: string syntax is already used for an object with a single property named number.

number: string

number

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