// http://stackoverflow.com/a/35101824/1238344

​

function frequencies(/* {Array} */ a){

    return new Map([...new Set(a)].map(

        x => [x, a.filter(y => y === x).length]

    ));

}

​

frequencies.identifiedBy = function(a, ...keys){

    var normalized = [];

    a.forEach(obj => {

        var identity = keys.map(key => obj[key]);

        var first = normalized.find(candidate => {

            var candidateId = keys.map(key => candidate[key]);

            return !identity.reduce((unequal, val, i) => unequal || candidateId[i] !== val, false);

        });

        normalized.push(first || keys.reduce((o, key) => (o[key] = obj[key], o), {}));

    });

    return frequencies(normalized);

}

​

// ---

​

var similarObjects = [

    { id: 0, value: 'foo', name: 'apple ' },

    { id: 1, value: 'bar', name: 'orange' }, 

    { id: 0, value: 'foo', name: 'apple ' },

    { id: 2, value: 'baz', name: 'banana' },

    { id: 3, value: 'foo', name: 'apple ' },

    { id: 4, value: 'baz', name: 'apple ' }

];

​

var logger = (freq, o) => {

    var values = Object.keys(o).map(key => o[key]);

    console.log(`(${values.join(', ')}): ${freq}`);

};

​

frequencies.identifiedBy(similarObjects, 'id').forEach(logger); 

frequencies.identifiedBy(similarObjects, 'value').forEach(logger);

frequencies.identifiedBy(similarObjects, 'value', 'name').forEach(logger);