jquery array group by

Here’s a plain javascript way to do it that collects the unique indexes in a map and totals them as it does, then regenerates the array with the totals:

abcArr = [["A", 10], ["B", 20], ["A",30],["C",40]];

var items = {}, base, key;
for (var i = 0; i < abcArr.length; i++) {
    base = abcArr[i];
    key = base[0];
    // if not already present in the map, add a zeroed item in the map
    if (!items[key]) {
        items[key] = 0;
    }
    // add new item to the map entry
    items[key] += base[1];
}

// Now, generate new array
var outputArr = [], temp;
for (key in items) {
    // create array entry for the map value
    temp = [key, items[key]]
    // put array entry into the output array
    outputArr.push(temp);
}

// outputArr contains the result

Working demo: http://jsfiddle.net/jfriend00/vPMwu/

---

Here’s a way using jQuery’s .each:

abcArr = [["A", 10], ["B", 20], ["A",30],["C",40]];

var items = {}, base, key;
$.each(abcArr, function(index, val) {
    key = val[0];
    if (!items[key]) {
        items[key] = 0;
    }
    items[key] += val[1];
});

var outputArr = [];
$.each(items, function(key, val) {
    outputArr.push([key, val]);
});

// outputArr contains the result
document.body.innerHTML = JSON.stringify(outputArr);

Working demo: http://jsfiddle.net/jfriend00/Q8LLT/