How to get most frequent element on a string array in typescript

Question

I have the following object:

export namespace Data {
  export type AsObject = {
    a: string,
    b: string,
    c: string,
    d: number,
  }
}

And I need to find the most frequent string Data.AsObject.a from Data.AsObject[]. Can someone please share a code in typescript to do it ? Thanks,

Answer 1

You didn't explain what should happen in the case that the array is empty or that there is a tie for greatest occurrence, so I implemented it with these conditions:

• an empty array will throw an error
• a tie will result in only one being returned (the one which occurs first)

You'll need to modify it if you want to change the behavior of either of those.

It's also implemented so that you can find the greatest occurrence of any other prop: just change the specified key as the second argument.

TS Playground

namespace Data {
  export type AsObject = {
    a: string;
    b: string;
    c: string;
    d: number;
  };
}

// If there's a tie, only one is returned
function greatestOccurrence <K extends keyof Data.AsObject>(
  arr: Data.AsObject[],
  key: K,
): Data.AsObject[K] {
  type T = Data.AsObject[K];
  const map = new Map<T, number>();

  for (const o of arr) {
    const item = o[key];
    map.set(item, (map.get(item) ?? 0) + 1);
  }

  const result = [...map].sort(([, a], [, b]) => b - a)[0]?.[0];
  if (typeof result === 'undefined') throw new Error('Array is empty');
  return result;
}

///// Use like this:

// I'll skip filling out the extra props in this input example:
const items = [
  {a: 'hello'},
  {a: 'hello'},
  {a: 'hola'},
  {a: 'willkommen'},
  {a: 'bonjour'},
  {a: 'hola'},
  {a: 'hola'},
] as Data.AsObject[];

const result = greatestOccurrence(items, 'a');
console.log(result); // "hola"