index-signature.md

Index signature is used to represent the type of object/dictionary when the values of the object are of consistent types.
Index signature is also known as keyed collection.

// Syntax
{ [key: typeForKeys]: typeForValues }

interface Dictionary {
    [index: string]: string;
}

let phoneBook: Dictionary;

phoneBook = {
    "Alice": "123-456-7890",
    "Bob": "987-654-3210",
    "Charlie": "456-789-0123"
};

let aliceNumber: string = phoneBook["Alice"]; // "123-456-7890"

These values will be consistent with the type string. This makes a perfect opportunity for us to apply the index signature.
What if we try to add a value of a different type other than the type defined in the index signature?

interface StringDictionary {
    [index: string]: string;
}

let myDict: StringDictionary;

myDict = {
    "name": "Alice",
    "age": "25"
};

// This will cause an error because the value is not a string
myDict["height"] = 170; // Error: Type 'number' is not assignable 

but if we create a union type with number and string, it will work.

interface StringDictionary {
    [index: string]: string | number;
}

let myDict: StringDictionary;

myDict = {
    "name": "Alice",
    "age": "25"
};

myDict["height"] = 170; 

Remember: Index signature parameter type must be string, number, symbol, or a template literal type, otherwise TypeScript will give an error.