Array methods for Dicts (Objects) - TypeScript

array-methods-for-dicts.ts

// From Frontend Masters Typescript Fundamentals v3 course

// Array.prototype.map, but for Dict
function mapDict<T, U>(
  input: Dict<T>,
  mappingCb: (arg: T, key: string) => U
): Dict<U> {
  const output: Dict<U> = {}

  // for-of loop is for an iterable
  // for-in loop iterates over the properties of an Object
  for (let key in input) {
    const thisVal = input[key]
    output[key] = mappingCb(thisVal, key)
  }

  return output
}

// Array.prototype.filter, but for Dict
function filterDict<T>(
  input: Dict<T>, 
  filterCb: (arg: T) => boolean): Dict<T> {
  const output: Dict<T> = {}

  for (let key in input) {
    const thisVal = input[key]
    if (filterCb(thisVal)) output[key] = thisVal
  }

  return output
}

// Array.prototype.reduce, but for Dict
function reduceDict<T, V>(
  input: Dict<T>,
  reducer: (currentValue: V, item: T) => V,
  initialVal: V
): V {
  let output = initialVal

  for (let key in input) {
    const thisVal = input[key]
    output = reducer(output, thisVal)
  }

  return output
}