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.

How about augmenting the built-in Array object to use Math.max/Math.min instead:

Array.prototype.max = function() {
  return Math.max.apply(null, this);
};

Array.prototype.min = function() {
  return Math.min.apply(null, this);
};

let p = [35,2,65,7,8,9,12,121,33,99];

console.log(`Max value is: ${p.max()}` +
  `\nMin value is: ${p.min()}`);

Here is a JSFiddle.

Augmenting the built-ins can cause collisions with other libraries (some see), so you may be more comfortable with just apply'ing Math.xxx() to your array directly:

var min = Math.min.apply(null, arr),
    max = Math.max.apply(null, arr);

Alternately, assuming your browser supports ECMAScript 6, you can use spread syntax which functions similarly to the apply method:

var min = Math.min( ...arr ),
    max = Math.max( ...arr );

Can you use Math.max with an array?

No, but...

If you're using Java 8, you can use streams:

Arrays.stream(array).max().getAsInt()

Otherwise you can write a simple utility method to do it for you:

public static int max(int... array) {
    if (array.length == 0) {
        // ...
    }

    int max = array[0];

    for (int a : array) {
        if (a > max)
            max = a;
    }

    return max;
}