javascript - group array items by

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const groupItems = (array, keyGetter) => {
    const access = {};  // lets to access group items quickly
    const groups = [];
    for (const item of array) {
        const key = keyGetter(item);
        if (key == null) {  // null or undefined
            continue;
        }
        const items = access[key];
        if (items) {
            items.push(item);
        } else {
            const items = [item];
            access[key] = items;
            groups.push({key, items});
        }
    }
    return groups;
};


// Usage example:

const users = [
    {id: 1, name: 'John', age: 21},
    {id: 2, name: 'Kate', age: 23},
    {id: 3, name: 'Matt', age: 21},
];
const groups = groupItems(users, (user) => user.age);  // groups users by age

console.log(groups);


// Output:
//
//     [
//         {
//             key: 21,
//             items: [
//                 {id: 1, name: 'John', age: 21},
//                 {id: 3, name: 'Matt', age: 21}
//             ]
//         },
//         {
//             key: 23,
//             items: [
//                 {id: 2, name: 'Kate', age: 23}
//             ]
//         }
//     ]

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const groupItems = (array, keyGetter) => {
    const groups = {};
    for (const item of array) {
        const key = keyGetter(item);
        if (key == null) {  // null or undefined
            continue;
        }
        const group = groups[key] ?? (groups[key] = []);
        group.push(item);
    }
    return groups;
};


// Usage example:

const users = [
    {id: 1, name: 'John', age: 21},
    {id: 2, name: 'Kate', age: 23},
    {id: 3, name: 'Matt', age: 21},
];
const groups = groupItems(users, (user) => user.age);  // groups users by age

console.log(groups);


// Output:
//
//     {
//         '21': [
//             {id: 1, name: 'John', age: 21},
//             {id: 3, name: 'Matt', age: 21}
//         ],
//         '23': [
//             {id: 2, name: 'Kate', age: 23}
//         ]
//     }


// See also:
//
//     1. https://dirask.com/snippets/JavaScript-group-array-items-by-pJZb9p

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const groupItems = (array, keyGetter) => {
    const reducer = (groups, item) => {
        const key = keyGetter(item);
        if (key != null) {  // not null and not undefined
            const group = groups[key] ?? (groups[key] = []);
            group.push(item);
        }
        return groups;
    };
    return array.reduce(reducer, {});
};


// Usage example:

const users = [
    {id: 1, name: 'John', age: 21},
    {id: 2, name: 'Kate', age: 23},
    {id: 3, name: 'Matt', age: 21},
];
const groups = groupItems(users, (user) => user.age);  // groups users by age

console.log(groups);


// Output:
//
//     {
//         '21': [
//             {id: 1, name: 'John', age: 21},
//             {id: 3, name: 'Matt', age: 21}
//         ],
//         '23': [
//             {id: 2, name: 'Kate', age: 23}
//         ]
//     }


// See also:
//
//     1. https://dirask.com/snippets/JavaScript-group-array-items-by-pJZb9p

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// Warning: Object.groupBy() method was introduced in 2023 in the major web browsers, Node.js and Deno.


const users = [
    {id: 1, name: 'John', age: 21},
    {id: 2, name: 'Kate', age: 23},
    {id: 3, name: 'Matt', age: 21},
];
const groups = Object.groupBy(users, (user) => user.age);  // groups users by age

console.log(groups);


// Output:
//
//     {
//         '21': [
//             {id: 1, name: 'John', age: 21},
//             {id: 3, name: 'Matt', age: 21}
//         ],
//         '23': [
//             {id: 2, name: 'Kate', age: 23}
//         ]
//     }


// See also:
//
//     1. https://dirask.com/snippets/JavaScript-group-array-items-by-pJZb9p