# TypeScript --- FAC 17 (Nov 2019)➡️ Freelance until Sep 2020 ➡️ Heka until March 2022 ➡️ Unmind June 2023 ➡️ TravelPerk Oct 2023 ⭐️ --- ## 4 stages of competence  --- The 3 stages of typescript grief 😖🙄😍 --- # 1  --- # 2  --- # 3  --- ## Types (Strings, Numbers, Boolean, Null, Undefined, BigInt, Symbol, <span style="color:blue">Object</span>) --- Javascript is an interpreted (or just-in-time compiled) **dynamic** (not static) and **weakly** (not strongly) typed language --- ## Static vs Dynamic "Statically-typed programming languages do type checking at compile-time (TS), whereas dynamically-typed languages do type checks at runtime. (JS)" ([source](https://www.educative.io/answers/statically-v-dynamically-v-strongly-v-weakly-typed-languages)) --- ## Weak vs Strong "Weakly-typed languages make conversions between unrelated types implicitly (JS); whereas, strongly-typed languages don’t allow implicit conversions between unrelated types.(TS)" ([source](https://www.educative.io/answers/statically-v-dynamically-v-strongly-v-weakly-typed-languages)) --- "JavaScript provides language primitives like string and number, but it doesn’t check that you’ve consistently assigned these. TypeScript does." - Someone --- # Examples --- # Interesting Concepts --- # Duck typing "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck" If an object that has no defined type is "compatible" with a type definition then it's fine. --- # Generics --- ## How is TS run? - JS is usually run in JIT compilation - TS must be compiled into JS to then be run - When compilation happens types are removed so these aren't accessible at run time! "X only refers to a type, but is being used as a value here" --- ## ts.config "The presence of a tsconfig.json file in a directory indicates that the directory is the root of a TypeScript project. The tsconfig.json file specifies the root files and the compiler options required to compile the project." --- ## Things to avoid <img src="https://hackmd.io/_uploads/rJHDAEHfT.jpg" alt="drawing" width="400"/> --- Using ```any``` ```typescript let something: any = "Hello World!"; something = 23; something = true; ``` --- Avoid type assertion e.g. ```typescript const name = "tony" as number; console.log(add(name, 2)); ``` --- Make sure you're using the lowercase versions of types e.g. `string` not `String` --- ## Almost done... --- # Why a presentation on TypeScript? --- ## Questions? ## FAQ - How does duck typing work? - This is because TypeScript uses structural typing (not nominative), which relies on the shape of the objects rather than their explicit class names. - Types vs interfaces - it depends - We can use define a type alias for a primitive type: type userName = string | undefined, const myAddress: userName = "tonylomax" . Can't do that with interfaces - Can only do union types with types e.g. type Transport = 'Bus' | 'Car' | 'Bike' | 'Walk';