Typescript custom array methods with generics. this is a exercise to understand how generics work and where to use them in real life

TS-generics-custom-array-every-method.ts

interface Array<T> {
  customEvery(callback: (each: T, index?: number, array?: Array<T>) => boolean): boolean;
}

Array.prototype.customEvery = function(callback) {
  let res= []
  for (let i = 0; i < this.length; i++) res.push(callback(this[i], i, this));
  return res.includes(false) ? false : true
}

const a = [1, 2, 3, 4, 5].customEvery((each)=>each > 1);
const b = [2, 3, 4, 5, 6].customEvery((each)=>each > 1);

console.log(a) // output: false
console.log(b) // output: true

TS-generics-custom-array-find-method.ts

interface Array<T> {
    customFind(matcher: (each: T, index?: number, array?: Array<T>)=> boolean): T | undefined;
}

Array.prototype.customFind = function(matcher) {
  for (let i = 0; i < this.length; i++) if(matcher(this[i], i, this)) return this[i]
}

const array: number[] = [1,2,3,4,5,6,7]
console.log(array.customFind((each)=>each===1)) // output: 1
console.log(array.customFind((each)=>each===11)) // output: undefined

TS-generics-custom-array-some-method.ts

interface Array<T> {
  customSome(callback: (each: T, index?: number, array?: Array<T>) => boolean): boolean;
}

Array.prototype.customSome = function (callback) {
  let res = []
  for (let i = 0; i < this.length; i++) res.push(callback(this[i], i, this));
  return res.includes(true) ? true : false
}

const a = [1, 1, 1, 1, 1].customSome((each) => each > 1);
const b = [1, 2, 3, 4, 5, 6].customSome((each) => each > 1);

console.log(a) // output: false
console.log(b) // output: true

TS-generics-custom-reducer.ts

interface Array<T> {
  customReduce<TElement, TResult>(reducer: (prev: TResult, curr: TElement, index?: number, array?: Array<T>) => TResult, initVal: TResult): TResult;
}

Array.prototype.customReduce = function(reducer, initial) {
  let result = initial;
  for (let elem of this) result = reducer(result, elem);
  return result
}

const rr = [1, 2, 3, 4, 5].customReduce<number, {[key: string]: number}>((prev, curr) => {
  if (!prev[`-${curr}-`]) prev[`-${curr}-`] = curr
  return prev
}, {});

TS-getters-keyof.ts

interface MyInterface {
  id: number;
  name: string;
  properties: string[];
}

const myObject: MyInterface = {
  id: 1,
  name: 'foo',
  properties: ['a', 'b', 'c']
};

function getValue(value: keyof MyInterface) {
  return myObject[value];
}

getValue('id'); // 1
getValue('count') // Throws compilation error: Argument of type '"count"' is not assignable to parameter of type '"id" | "name" | "properties"'.