How do i use Math.max with an array of objects?

One more way, I think it is more beautiful. By using spread:

Math.max(...array.map((i) => i.x));

You'll have to extract the properties from the objects yourself:

var maximum = Math.max.apply(Math, myArr.map(function(o) { return o.x; }));

That uses .map() to iterate through the elements of the array and return the value of the "x" property. That result array is then passed as the arguments to Math.max().

Now that => functions are widely available it'd be a little shorter as so:

var maximum = Math.max.apply(Math, myArr.map(o => o.x));

Still doing pretty much the exact same thing of course.

var myArr = [{x:100,y:200},{x:200,y:500}];

function sortByKeyDesc(array, key) {
    return array.sort(function(a, b) {
        var x = a[key]; var y = b[key];
        return ((x < y) ? 1 : ((x > y) ? -1 : 0));
    });
}
console.log(sortByKeyDesc(myArr, "x")[0].x);

first sort by key value in descending order and then get the first object's x property