How do I find in an array of objects an object that doesnt have a property 'x'?

I'd be inclined to use Object.prototype.hasOwnProperty[^] to determine whether the property has been defined:

var value = objects.find(function(el){ return !el.hasOwnProperty("age"); });

NB: Array.prototype.find[^] will find the first element which matches. If you want to find all matching elements, use Array.prototype.filter[^] instead.

var allValues = objects.filter(function(el){ return !el.hasOwnProperty("age"); });

Note that both functions have limited support in older browsers. There are polyfills available.

If you're only supporting newer browsers - specifically not Internet Explorer - you might also want to consider using Arrow functions[^] to simplify the code:

var value = objects.find(el => !el.hasOwnProperty("age"));
var allValues = objects.filter(el => !el.hasOwnProperty("age"));