Group arrays by key using Array.prototype.reduce in Javascript

groupBy.js

 /**
     * Groups an array of objects by a key an returns an object or array grouped by provided key.
     * @param array - array to group objects by key.
     * @param key - key to group array objects by.
     * @param removeKey  - remove the key and it's value from the resulting object.
     * @param outputType - type of structure the output should be contained in.
     */
    const groupBy = (
      inputArray,
      key,
      removeKey = false,
      outputType = {},
    ) => {
      return inputArray.reduce(
        (previous, current) => {
          // Get the current value that matches the input key and remove the key value for it.
          const {
            [key]: keyValue
          } = current;
          // remove the key if option is set
          removeKey && keyValue && delete current[key];
          // If there is already an array for the user provided key use it else default to an empty array.
          const {
            [keyValue]: reducedValue = []
          } = previous;

          // Create a new object and return that merges the previous with the current object
          return Object.assign(previous, {
            [keyValue]: reducedValue.concat(current)
          });
        },
        // Replace the object here to an array to change output object to an array
        outputType,
      );
    };