Check Key Exists in Array

You seem to be mixing arrays and objects here.

if (test[office] == null)

will test whether the array test has a property abc, def etc. This condition will always be true because arrays don't have such properties. You usually add properties with numeric properties to an array, e.g. with .push like you do as well.

Objects on the other hand can have arbitrary properties.

Now regarding:

test["office"][office] += 1;

This accesses the property office of the array (which does not exist) and then the properties abc, def, etc (which of course don't exist either, since test.office does not exist in the first place).

You have to decide whether you want test to be an array or an object.
If you choose an object, aggregating the data will be easier since you can easily test for existence of an object property.
If you want an array, you can either convert the object to an array, or if you use one from the start, you'd have to iterate over the elements of the array and find the right one. That solution is less preferable, since the runtime will be O(n^2).

Object:

var test = {};

ko.utils.arrayForEach(officeLines, function (item, indx) {
    var office = item.Office;
    if (!test[office]) {
        hash[office] = 1
    }
    else {
        hash[office] += 1;
    }
});

test would then look like:

{
    abc: 1,
    def: 2,
    ghi: 1
}

and it would be easy to create an array from that like you want it.

Array:

var test = [];

ko.utils.arrayForEach(officeLines, function (item, indx) {
    var office = item.Office;
    for (var i = 0, l = test.length; i < l; i++) {
        if (test[i].office === office) {
            test[i].count += 1;
            return;
        }
    }
    test.push({office: office, count: 1});
});

I hope you can already see from the nested loop (for inside forEach) that this is not an optimal solution.

Further reading material:

• MDN - Working with Objects
• eloquentjavascript.net - Data structures: Objects and Arrays

What about something like this?

var test = {};
ko.utils.arrayForEach(officeLines, function (item, indx)
{
    var office = item.Office;
    if (typeof test[office] === "undefined")
    {
        test[office] = 1;
    }
    else
    {
        test[office]++;
    }
});
// back to array
var array = [];
for (var office in test) {
    array.push({ office: office, count: test[propertyName] });
}

I think you might not have to convert back to an array, because you might be able to use $index to get the office name, then $data for the count in a foreach in knockoutjs.

var officeLines = [{"Office":"abc", "Name":"ABC", "Total":0},
                   {"Office":"def", "Name":"DEF", "Total":11},
                   {"Office":"def", "Name":"DEF", "Total":1},
                   {"Office":"ghi", "Name":"GHI", "Total":1111}];

var test = {};

for (office_idx in officeLines) {
    var office = officeLines[office_idx];
    if (test[office['Office']]) {
        test[office['Office']]['count'] += 1;
    } else {
        test[office['Office']] = {office: office, count: 1};
    }
}
var results = []
for (office_id in test){
    results.push(test[office_id]);
}

How about instead of pushing the results to an array you push them to an object like so:

var officeLines = [{"Office":"abc", "Name":"ABC", "Total":0},
{"Office":"def", "Name":"DEF", "Total":11},
{"Office":"def", "Name":"DEF", "Total":1},
{"Office":"ghi", "Name":"GHI", "Total":1111}];
var offices = {};
ko.utils.arrayForEach(officeLines, function (item, indx)
{
    var office = item.Office;
    if (!offices[office])
    {

        offices[office] = 1;
    }
    else
    {
        offices[office]++;
    }
});

document.write(JSON.stringify(offices)); // -> {"abc":1,"def":2,"ghi":1}

http://jsfiddle.net/zd5Db/