TypeScript Note

Todo

• [ ] merge from dev diary TypeScript 1004 2021-10-12

class setter

• @2021-12-12
• Setter will not set value. It's like declaring a method rather than a real property of the class. setter -MDN

"OneOf" Operator

• @2021-12-12
• I want to make a number-like type N, such that the N can be either always number or always bigint. But it seems not possible to achieve with current TypeScript 4.5.3.

relate array and literal type

• @2021-12-11
• How to convert an array into string literal union type in TypeScript?
• const list = ["foo","bar"] as const; type litType = typeof list[number];, as const is the key here;

Array.prototype.at method

• @2021-12-11
• Support Array.prototype.at would come with ES2022 lib (from GitHub).
• @2022-01-03
• Supported by TS4.5.4

Firebase-js-SDK

• @2021-10-12
• firebase-js-sdk PR#5600
• In the context of a type for property of an interface, extends cannot be used. The &(intersection) operator has same effect.
• There is not requirement in TypeScript for an exact match between the implementing class field and the interface field.(origin)
• @2021-12-14
• Read Type theory: academic study of a formal system in which every term has a "type". Natural language is has a simpler(more concrete, less abstract) grammar than formal language.

Get static types

• @2021-10-14
• This is like excluding a literal type from the prototype.
• Use utility types Exclude<>
• With Exclude<keyof typeof CUSTOMIZED_CLASS, "prototype"> can get all static properties. here is new for me.

Module

• @2021-12-18
• Add property to current module: declare module
• Override property: TS: How to override properties with type intersection

Typing for ... in loop

• @2021-12-28
• use let key: KEYTYPE; to declare the type before the loop.
• The left-hand side of a 'for...in' statement cannot use a type annotation

Use file API

• @2021-12-28
• File System Access API
• need to add @types/wicg-file-system-access for TS

Writing produces type intersection

• @2022-01-03
• The problem was encountered during the refactor process of AbstrLabs.
• Writing to a number|string type will cause the type narrow to never by intersection.
• Details:
  • Gist
  • Issue in repo
  • Stack Overflow

Type equality

• @2022-01-11
• These are untested resources.
• How to test if two types are exactly the same
• This post of the issue type level equal operator worths a try.

Destructuring assignment with condition

• @2022-01-11
• Writing declare var then change it is not a good pattern.
• Use var _tmp for the condition like if, switch, then assign from _tmp

ES2021 New Features

• @2022-02-04
• Also called ES12.
• String.prototype.replaceAll()
• WeakRef and FinalizationRegistry
• Promise.any() possibly throws an AggregateError
• Logical Assignment Operator &&, || and ??. In the original webpage, it's not explained very clearly. x&&=y is same as x=x&&y.
• Underscores as Numeric Separator. Like Python 1_000_000
• All from Backbencher.dev