A more readable and annotated version of the Javascript groupBy from Ceasar Bautista (https://stackoverflow.com/a/34890276/1376063)

groupBy.js

var groupBy = function(data, key) { // `data` is an array of objects, `key` is the key (or property accessor) to group by
  // reduce runs this anonymous function on each element of `data` (the `item` parameter,
  // returning the `storage` parameter at the end
  return data.reduce(function(storage, item) {
    // get the first instance of the key by which we're grouping
    var group = item[key];
    
    // set `storage` for this instance of group to the outer scope (if not empty) or initialize it
    storage[group] = storage[group] || [];
    
    // add this item to its group within `storage`
    storage[group].push(item);
    
    // return the updated storage to the reduce function, which will then loop through the next 
    return storage; 
  }, {}); // {} is the initial value of the storage
};

To make this more readable, I renamed the parameters, and split up one line into several to get a better model of what's happening.
I changed (rv[x[key]] = rv[x[key]] || []).push(x); to:

var group = item[key];
storage[group] = storage[group] || [];
storage[group].push(item);

How would you introduce multiple keys to group by?

How would you introduce multiple keys to group by?

var groupBy = function (xs, key1, key2) {
    return xs.reduce(function (rv, x) {
        (rv[x[key1] + ',' + x[key2]] = rv[x[key1] + ',' + x[key2]] || []).push(x);
        return rv;
    }, {});
};

this piece of code can do the job

Thank you SO MUCH, for creating this readable version of the code. It helps me a lot.<3

Just spent significant amount of time trying to understand this before I noticed your comment in Stack Overflow. Very useful, Thank you!

Thanks for sharing this! I've created another version that supports multiple keys https://gist.github.com/one89/395a3ffb2df4d0648ec7b0caf6cf5d37

Thank you for this. Very helpful

How would you introduce multiple keys to group by?

I'm using a version of this that allows supplying any list of keys (granted that they're all strings). You can rewrite the function supplied to keys.map to handle non-string values fairly easily:

/**
 * @param {any[]} arr
 * @param {string[]} keys
 */
const groupBy = (arr, keys) => {
  return arr.reduce((storage, item) => {
    const objKey = keys.map(key => `${ item[key] }`).join(':'); //should be some unique delimiter that wont appear in your keys
    if (storage[objKey]) {
      storage[objKey].push(item);
    } else {
      storage[objKey] = [item];
    }
    return storage;
  }, {});
};

It would be great to have a TypeScript version of this.

It would be great to have a TypeScript version of this.

@DarkLite1 This is how I've adapted it for TS use:

const groupBy = <T>(arr: T[], keys: (keyof T)[]): { [key: string]: T[] } => {
  return arr.reduce((storage, item) => {
    const objKey = keys.map(key => `${ item[key] }`).join(':');
    if (storage[objKey]) {
      storage[objKey].push(item);
    } else {
      storage[objKey] = [item];
    }
    return storage;
  }, {} as { [key: string]: T[] });
}

Thank you for getting back to me. On StackOvelfow we posted the same question and got this answer:

type ObjectKey = string | number | symbol

export const groupBy = <
  K extends ObjectKey,
  TItem extends Record<K, ObjectKey>
>(
  items: TItem[],
  key: K
): Record<ObjectKey, TItem[]> =>
  items.reduce(
    (result, item) => ({
      ...result,
      [item[key]]: [...(result[item[key]] || []), item],
    }),
    {} as Record<ObjectKey, TItem[]>
  )

I'm still a noob, so I don't know which one is better but it works fine. The only thing to keep in mind is that the value for ObjectKey cannot be empty. It would be great if it was empty that it would use a default value to group by.

The answer you got on SO is more complete, mine is a more naive approach that assumes objects in the array have no nested objects internally and that keys are always strings, though the SO answer does restrict you to a single key (you can pretty easily change that to make the second argument into the function an array of keys or just do a spread of string values for all other function arguments). If you need this for a more simple use case you could use my approach which doesn’t require you to specify key typing

thanks,really helpfull