Group array items using object

by

Start by creating a mapping of group names to values.
Then transform into your desired format.

var myArray = [
    {group: "one", color: "red"},
    {group: "two", color: "blue"},
    {group: "one", color: "green"},
    {group: "one", color: "black"}
];

var group_to_values = myArray.reduce(function (obj, item) {
    obj[item.group] = obj[item.group] || [];
    obj[item.group].push(item.color);
    return obj;
}, {});

var groups = Object.keys(group_to_values).map(function (key) {
    return {group: key, color: group_to_values[key]};
});

var pre = document.createElement("pre");
pre.innerHTML = "groups:\n\n" + JSON.stringify(groups, null, 4);
document.body.appendChild(pre);

Using Array instance methods such as reduce and map gives you powerful higher-level constructs that can save you a lot of the pain of looping manually.

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