This document summarizes TypeScript features such as type checking, refactoring, type guards, discriminated unions, keyof operator, Pick, Exclude, OmitType and configuration options. It was presented by Jussi Haapanen at the TampereJS meetup on October 11, 2018. TypeScript adds optional static typing to JavaScript and is developed and maintained by Microsoft.

1. Awesome TypeScript
Jussi Haapanen
TampereJS meetup
11.10.2018

2. What is
• ”TypeScript is an open-source programming language developed and
maintained by Microsoft. It is a strict
syntactical superset of JavaScript, and adds optional static typing to
the language.” [1]

3. Used by:

4. TypeScript features
• Type checking
• Refactoring

5. Type guards
• Let’s you specify a function that returns true/false based on whether
the passed parameter is of a specific type

6. Discriminated union
• Sort of does the same thing as above but less boilerplate
• Compile knows which type object is (out of a set of types) based on a
common field

7. keyof T operator
• Returns a set of permitted property names for object T
• E.g. keyof Person  ”firstName” | ”lastName”

8. Pick<T, K>, Exclude<T, K>, OmitType<T1,T2>,
...
• Useful if you need to add or remove missing fields from a type
• Pick and Exclude and others are provided by the language
• OmitType requires a custom type
• export type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
• export type OmitType<T1 extends T2, T2> = Pick<T1, Exclude<keyof
T1, keyof T2>>;

9. Configuration
• downLevelIteration
• Async/Await, Spread operation support for ES3
• strictNullChecks
• Makes sure you can’t mix type T, undefined or null

10. Questions?

11. References
• [1] https://en.wikipedia.org/wiki/TypeScript