# Typescript - Tips & Tricks - Index Signature

> Welcome back guys, today I'll speak about the "Index Signature". In some cases, we need to create...

- **Author**: Luca Del Puppo (@puppo92) — AI Native Engineer at Nearform
- **Published**: Mar 5, 2021
- **Canonical URL**: https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-index-signature/
- **Series**: TypeScript - Tips & Tricks (Part 9 of 21) — https://delpuppo.net/blog/series/ts-tips-and-tricks/
- **Series LLMs.txt**: https://delpuppo.net/blog/series/ts-tips-and-tricks/llms.txt
- **Category**: TypeScript
- **Tags**: typescript, webdev
- **Hashnode Mirror**: https://blog.delpuppo.net/typescript-tips-tricks-index-signature
- **Dev.to Mirror**: https://dev.to/playfulprogramming/typescript-tips-tricks-index-signature-2og6
- **Blog Index LLMs.txt**: https://delpuppo.net/blog/llms.txt

## Series Context: TypeScript - Tips & Tricks

- Part 1: [Typescript - Tips & Tricks - typeof](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-typeof/)
- Part 2: [Typescript - Tips & Tricks - Literal Types](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-literal-types/)
- Part 3: [Typescript - Tips & Tricks - keyof](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-keyof/)
- Part 4: [Typescript - Tips & Tricks - Union and Intersection](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-union-and-intersection/)
- Part 5: [Typescript - Tips & Tricks - Overloaded Functions](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-overloaded-functions/)
- Part 6: [Typescript - Tips & Tricks - this parameter](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-this-parameter/)
- Part 7: [Typescript - Tips & Tricks - any vs. unknown](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-any-vs-unknown/)
- Part 8: [Typescript - Tips & Tricks - Optional modifier](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-optional-modifier/)
- Part 9: [Typescript - Tips & Tricks - Index Signature](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-index-signature/) **(Current Article)**
- Part 10: [Typescript - Tips & Tricks - readonly modifier](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-readonly-modifier/)
- Part 11: [Typescript - Tips & Tricks - Readonly Array](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-readonly-array/)
- Part 12: [Typescript - Tips & Tricks - Tuple](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-tuple/)
- Part 13: [Typescript - Tips & Tricks - Non-null assertion operator](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-non-null-assertion-operator/)
- Part 14: [Typescript - Tips & Tricks - Never type](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-never-type/)
- Part 15: [Typescript - Tips & Tricks - Assert Function](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-assert-function/)
- Part 16: [Typescript - Tips & Tricks - Type Guard](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-type-guard/)
- Part 17: [Typescript - Tips & Tricks - Conditional Type](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-conditional-type/)
- Part 18: [Typescript - Tips & Tricks - infer keyword](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-infer-keyword/)
- Part 19: [Typescript - Tips & Tricks - Mapped Types](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-mapped-types/)
- Part 20: [Typescript - Tips & Tricks - Advanced Types](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-advanced-types/)
- Part 21: [Typescript - Tips & Tricks - Rest Parameters & Rest Arguments](https://delpuppo.net/blog/ts-tips-and-tricks/typescript-tips-tricks-rest-parameters-rest-arguments/)

---

Welcome back guys, today I'll speak about the _"Index Signature"_.
In some cases, we need to create some special types like dictionaries.
These special types have some keys that identifies the elements and the datas.
A simple example:
```ts
export type User = {
    name: string,
    email: string;
    session: string
}

export type UserDictionary = {
    [username: string]: User
}

const userDictionary: UserDictionary = {
  'myusername': { email: "myemail@email.it", name: "myname", session: "session" },
  'myusername1': {
    email: "myemail1@email.it",
    name: "myname1",
    session: "session",
  },
};

console.log(userDictionary.myusername); // { email: 'myemail@email.it', name: 'myname', session: 'session' }
console.log(userDictionary["myusername"]); // { email: 'myemail@email.it', name: 'myname', session: 'session' }
console.log(userDictionary.myusername1); // { email: 'myemail1@email.it', name: 'myname1', session: 'session' }
console.log(userDictionary["myusername1"]); // { email: 'myemail1@email.it', name: 'myname1', session: 'session' }
delete userDictionary.myusername;

```
In this case, the _UserDictionary_ is a special type where the usernames are the keys of the objects and the user data are the values.
This type is powerful because it permits the consumer to access directly to the data if it knows the keys and it permits to store a unique value of a specific key.
With the index signature, we can create special types where the keys can be string or number.
An important thing that you must remember is that the keys of these objects can be iterated with the for-in loop or with the Object.keys method.
```ts
console.log(Object.keys(userDictionary)); // [ 'myusername', 'myusername1' ]
for (const key in userDictionary) {
  console.log(key);
}
/*
'myusername'
'myusername1'
*/
```

It's all from the _Index Signature_ for today.
Bye-bye guys!