https://github.com/faruktoptas/typescript-tips
https://github.com/faruktoptas/typescript-tips
Last synced: 7 months ago
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- Host: GitHub
- URL: https://github.com/faruktoptas/typescript-tips
- Owner: faruktoptas
- Created: 2022-11-26T09:35:59.000Z (almost 3 years ago)
- Default Branch: main
- Last Pushed: 2022-12-06T15:37:06.000Z (almost 3 years ago)
- Last Synced: 2025-01-13T19:39:22.846Z (9 months ago)
- Language: TypeScript
- Size: 34.2 KB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
## Equality
`== undefined` or `== null` checks whether a value is either null or undefined.
```ts
let a;
let b = null
console.log(a,b);
if (a == null) {
console.log('a is null');
}
if (a == undefined) {
console.log('a is null');
}
if (b == null) {
console.log('b is null');
}
if (b == undefined) {
console.log('b is null');
}
```
## `in` operator narrowing
```ts
type Fish = { swim: () => void };
type Bird = { fly: () => void };
function move(animal: Fish | Bird) {
if ("swim" in animal) {
return animal.swim();
}
return animal.fly();
}
```
## Exhaustiveness checking
```ts
type Shape = Circle | Square;
function getArea(shape: Shape) {
switch (shape.kind) {
case "circle":
return Math.PI * shape.radius ** 2;
case "square":
return shape.sideLength ** 2;
default:
const _exhaustiveCheck: never = shape;
return _exhaustiveCheck;
}
}
```
## Optional types
```ts
function setPadding(side: 'top' | 'bottom' | 'left' | 'right', padding: number) {
// ...
}
```
```ts
type someFunction = (a: number, b: number) => number;
type SomeType = {
name: string;
calculate?: someFunction;
}
const someObject: SomeType = {
name: 'Some Object',
calculate: (a: number, b: number) => a + b
}
if (someObject.calculate) {
console.log(someObject.calculate(1, 2));
} else {
console.log('No calculate function');
}
```
## Call Signatures
Something callable with properties
```ts
type DescribableFunction = {
description: string;
(someArg: number): boolean;
};
function doSomething(fn: DescribableFunction) {
console.log(fn.description + " returned " + fn(6));
}
```
## Generic constraints
```ts
function longest(a: Type, b: Type) {
if (a.length >= b.length) {
return a;
} else {
return b;
}
}
// longerArray is of type 'number[]'
const longerArray = longest([1, 2], [1, 2, 3]);
// longerString is of type 'alice' | 'bob'
const longerString = longest("alice", "bob");
// Error! Numbers don't have a 'length' property
const notOK = longest(10, 100);
```
## Extensions
Extending a type
```ts
declare global {
interface Number {
thousandsSeperator(): String;
}
interface String {
hello(): number
}
}
Number.prototype.thousandsSeperator = function (): string {
return Number(this).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ',');
}
String.prototype.hello = function (): number {
return String(this).length + 100;
}
export { };
```
`main.ts`
```ts
import './ext'
const nu = 123456;
console.log(nu.thousandsSeperator()) // 123,456
console.log("Hello".hello()); // 105
```