Typescript cheatsheet

Starting

• install
• npm install TypeScript
• initialize, creates tsconfig.json
• npx tsc --init
• build
• npx tsc
• run
• npx ts-node index.ts

Basics

• variable declaration
• let, const
• NOT var
• variable testing
• typeof foo -> string, number, bigint, boolean, symbol, undefined, object, function
• instance of Foo -> true if foo is an instance of Foo
• null-ish stuff
• ?? -> foo ?? 'default' -> returns foo it's set, otherwise returns 'default'
• ! -> getFoo()! -> asserts that getFoo() will not return null or undefined
• basic types
• basics: string, number, boolean, undefined, null, void, any, unknown, never (skipped) ts const typeString: string = "This is a string"; const typeNumber: number = 42; const typeBoolean: boolean = true; const typeUndefined: undefined = undefined; const typeNull: null = null; const typeVoid: void = undefined; // void is often used for functions that do not return a value const typeUnknown: unknown = "This could be anything";

• arrays: string[], number[], boolean[] ```ts const stringArray: string[] = ["apple", "banana", "cherry"]; const numberArray: number[] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; const booleanArray: boolean[] = [true, false, true];

  let sub: number[] = numberArray.slice(1, 6); // Get a sub-array from index 1 to 5 (not including index 6) sub.push(11); // Add 11 to the end of the array sub = sub.filter(num => num % 2 === 0); // Filter even numbers sub.splice(2, 1, 12); // Replace the third element with 12 sub.shift(); // Remove the first element sub = sub.concat([13, 14]); // Concatenate with another array const arraySum = sub.reduce((sum, curr) => sum + curr, 0); // Sum of elements - object - `{ id: number; name: string }` - function - `const len = (param: string): number => { return param.length; }` - `const len = (param: string) => param.length;` - tuple - `[string, number]`ts const tuple: [string, number] = ["apple", 5]; const [fruit, count] = tuple; - Datets const now = new Date(); const yesterdayGD = new Date(now); yesterdayGD.setDate(now.getDate() - 1); const yesterdayGT = new Date(now.getTime() - 24 * 60 * 60 * 1000); const durationMs = now.getTime() - yesterdayGD.getTime(); - Mapts const map: Map = new Map([ ["a", 1], ["b", 2], ["c", 3] ]); map.get("a"); // returns 1 map.set("d", 4); map.has("b"); // returns true map.delete("c"); // removes key "c" map.clear(); // removes all entries - Setts const set: Set = new Set([1, 2, 3]); set.add(4); set.add(2); // Duplicate, will not be added set.delete(1); - Regexpts const regex = /p=(\d+),(\d+) v=(-?\d+),(-?\d+)/; const match = regex.exec("p=1,2 v=-3,4")!; const pos = { x: parseInt(match[1]!, 10), y: parseInt(match[2]!, 10) }; const vel = { x: parseInt(match[3]!, 10), y: parseInt(match[4]!, 10) }; - interfaces - example, interfacets interface Info { id: number; name: string; coords: { x: number; y: number; }; expiration?: Date; tags: string[] | null;

  getInfo: () => string;
  findTag(tag: string): boolean;
  
  get size(): number;
  set size(value: number | string);
  

  } - example, initializationts const info: Info = { id: 1, name: "Info Object", coords: { x: 10, y: 20 }, expiration: new Date(), tags: ["tag1", "tag2"],

  getInfo() {
      return `ID: ${this.id}, Name: ${this.name}, Coords: (${this.coords.x}, ${this.coords.y})`;
  },
  
  findTag(tag: string): boolean {
      return this.tags ? this.tags.includes(tag) : false;
  },
  
  get size(): number {
      return this.tags ? this.tags.length : 0;
  },
  set size(value: number | string) {
      if (typeof value === "number") {
          this.tags = Array(value).fill("defaultTag");
      } else if (typeof value === "string") {
          this.tags = [value];
      }
  }
  

  }; - generics - examplets interface DataSource { fetchData(): ResponseType; } interface DataSource2 { fetchData(): ResponseType; } ```