String or Number Indexes

The index signature can define members of an object with an identifier with square brackets.

myObject["memberName"] = "Value Here";

The identifier type can only be a string or a number. The identifier is the name between the brackets. In the previous example, it was memberName. In the following example, we can see an identifier with a string and another with a number;

myObject["String Here"] = {name:"anything"}; // string identifier
myObject[100] = true; // number identifier

Index signatures can be useful if you want to control the type of key for a dynamic assignment as well as the type to be returned.

For example, imagine having a custom map type that allows you to have a string key and return a specific type of object. Having a defined index signature allows for a strongly-typed key (string or number) that can hold an explicit type. In the following example, line 2 defines the type of index signature. The identifier must be a number while the value, a string. Line 4 demonstrates how to create a new object with two values with identifier 100 and 200.