JavaScript - group / grouping array items with reduce method

In this article, we're going to have a look at how to group array items with Array reduce() method. This article shows two ways to use Array reduce method to group array by item property. Second one example shows how to group it with nested properties.

1. Group by object property example

This section contains simple grouping of the array by property. Array reduce method in below example takes two arguments:

• function that converts array structure,
• empty object that will be filled with groups.

To change group name it is necessary only to change property variable.

// ONLINE-RUNNER:browser;

function groupItems(array, property) {
    var reducer = function(groups, item) {
        var name = item[property]
        var group = groups[name] || (groups[name] = []);
        group.push(item);
        return groups;
    };
    return array.reduce(reducer, {});
}


// Usage example:

var array = [
    {name: 'John',  age: 25},
    {name: 'Kate',  age: 21},
    {name: 'Pablo', age: 25},
    {name: 'Denis', age: 25},
    {name: 'Chris', age: 31}
];

var groups = groupItems(array, 'age');  // array will be grouped by 'age' property


console.log('[key]\t[group]');

for(var key in groups) {
    var group = groups[key];
    console.log('"' + key + '"\t' + JSON.stringify(group));
}

2. Group by nested object property example

In this section first example was modiffied to provide grouping by nested property.

To change group name it is necessary only to change path array to own one.

// ONLINE-RUNNER:browser;

function getValue(object, path) {
    var reducer = function(tmp, key) {
        return tmp[key];
    };
    return path.reduce(reducer, object);
}

function groupItems(array, path) {
    var reducer = function(groups, item) {
        var name = getValue(item, path);
        var group = groups[name] || (groups[name] = []);
        group.push(item);
        return groups;
    };
    return array.reduce(reducer, {});
}


// Usage example:

var path = ['details', 'age'];  // array items will be grouped by `details.age`

var array = [
    {name: 'John',  details: {age: 25}},
    {name: 'Kate',  details: {age: 21}},
    {name: 'Pablo', details: {age: 25}},
    {name: 'Denis', details: {age: 25}},
    {name: 'Chris', details: {age: 31}}
];

var groups = groupItems(array, path);


console.log('[key]\t[group]');

for(var key in groups) {
    var group = groups[key];
    console.log('"' + key + '"\t' + JSON.stringify(group));
}

See also

1. JavaScript - merge objects in array with the same keys

References

1. Array.prototype.reduce() - JavaScript | MDN