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
};

thank you all for these great solutions. Really helped me 😄

Thanks!

A simple version with a supplier for the groups

const groupBy = (arr, keysSupplier) => {
    return arr.reduce((obj, elt) => {
        let key = keysSupplier(elt)
        if (obj[key] === undefined)
            obj[key] = []
        obj[key].push(elt)
        return obj
    }, {})
}

// Example
// Grouping array by typeof
groupBy(array, elt => typeof elt)

i looking for these outputs
[
{
   custom: 'data1 custom b',
   group_by: 'school x',
   "children": {
       // data
   }
},
{
   custom: 'data2 custom a',
   group_by: 'school a',
   "children": {
       // data
   }
},
]

but the unexpected is

'school a': {
  // data
 },
'school x': {
  // data
}

i have tried with

storage['data'] = storage['data'] || [];
    storage[group] = storage[group] || [];

    // add this item to its group within `storage`
    storage[group].push(item);

    // storage['data'].push({
    //   label: group,
    //   mode: 'span',
    //   html: false,
    //   children: storage[group]
    // });

but its looping the result.

i have solved by add new function look like these

exports.groupByVue = async function(data)  {
  let result = [];
  for (const key in data) {
    if (Object.hasOwnProperty.call(data, key)) {
      const element = data[key];
      result.push({
        label: key,
        children: element,
      })
    }
  }
  return result;
}

but i want the code is inside the group by as default .