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groupBy

TS JS Deno

Groups the elements of an array based on the given function.

Use Array.prototype.map() to map the values of an array to a function or property name. Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.

typescript
type MapFunc<T = any> = (val: T, index?: number, arr?: T[]) => T;

const groupBy = <T = any>(arr: T[], fn: MapFunc<T> | string) =>
  arr.map(isString(fn) ? (val: any) => val[fn] : fn).reduce((acc, val, i) => {
    acc[val] = (acc[val] || []).concat(arr[i]);
    return acc;
  }, {});
typescript
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(["one", "two", "three"], "length"); // {3: ['one', 'two'], 5: ['three']}