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Merge duplicate objects in array of objects

I have below array of objects,

var data = [
    {
        label: "Book1",
        data: "US edition"
    },
    {
        label: "Book1",
        data: "UK edition"
    },
    {
        label: "Book2",
        data: "CAN edition"
    }
];

I want to merge the duplicate objects based on attribute ‘label’ so that Final output will look like below,

var data = [
    {
        label: "Book1",
        data: ["US edition", "UK edition"] //data attribute is merged
    },
    {
        label: "Book2",
        data: "CAN edition"
    }
];

Can someone help me identify the approach?

Answer

I would probably loop through with filter, keeping track of a map of objects I’d seen before, along these lines (edited to reflect your agreeing that yes, it makes sense to make (entry).data always an array):

var seen = {};
data = data.filter(function(entry) {
    var previous;

    // Have we seen this label before?
    if (seen.hasOwnProperty(entry.label)) {
        // Yes, grab it and add this data to it
        previous = seen[entry.label];
        previous.data.push(entry.data);

        // Don't keep this entry, we've merged it into the previous one
        return false;
    }

    // entry.data probably isn't an array; make it one for consistency
    if (!Array.isArray(entry.data)) {
        entry.data = [entry.data];
    }

    // Remember that we've seen it
    seen[entry.label] = entry;

    // Keep this one, we'll merge any others that match into it
    return true;
});

In an ES6 environment, I’d use seen = new Map() rather than seen = {}.

Note: Array.isArray was defined by ES5, so some quite older browsers like IE8 won’t have it. It can easily be shimmed/polyfilled, though:

if (!Array.isArray) {
    Array.isArray = (function() {
        var toString = Object.prototype.toString;
        return function(a) {
            return toString.call(a) === "[object Array]";
        };
    })();
}

Side note: I’d probably also always make entry.data an array, even if I didn’t see two values for it, because consistent data structures are easier to deal with. I didn’t do that above because your end result showed data being just a string when there was only one matching entry. (We’ve done that above now.)

Live example (ES5 version):

var data = [
    {
        label: "Book1",
        data: "US edition"
    },
    {
        label: "Book1",
        data: "UK edition"
    },
    {
        label: "Book2",
        data: "CAN edition"
    }
];
snippet.log("Before:");
snippet.log(JSON.stringify(data, null, 2), "pre");
var seen = {};
data = data.filter(function(entry) {
    var previous;

    // Have we seen this label before?
    if (seen.hasOwnProperty(entry.label)) {
        // Yes, grab it and add this data to it
        previous = seen[entry.label];
        previous.data.push(entry.data);

        // Don't keep this entry, we've merged it into the previous one
        return false;
    }

    // entry.data probably isn't an array; make it one for consistency
    if (!Array.isArray(entry.data)) {
        entry.data = [entry.data];
    }

    // Remember that we've seen it
    seen[entry.label] = entry;

    // Keep this one, we'll merge any others that match into it
    return true;
});
snippet.log("After:");
snippet.log(JSON.stringify(data, null, 2), "pre");
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