Testing properties

var o = { x: 1 }
"x" in o;         // true: o has an own property "x"
"y" in o;         // false: o doesn‘t have a property "y"
"toString" in o;  // true: o inherits a toString property

var o = { x: 1 }
o.hasOwnProperty("x");        // true: o has an own property x
o.hasOwnProperty("y");        // false: o doesn‘t have a property y
o.hasOwnProperty("toString"); // false: toString is an inherited property

var o = inherit({ y: 2 });
o.x = 1;
o.propertyIsEnumerable("x");  // true: o has an own enumerable property x
o.propertyIsEnumerable("y");  // false: y is inherited, not own
Object.prototype.propertyIsEnumerable("toString"); // false: not enumerable

var o = { x: undefined }   // Property is explicitly set to undefined
o.x !== undefined          // false: property exists but is undefined
o.y !== undefined          // false: property doesn‘t even exist
"x" in o                   // true: the property exists
"y" in o                   // false: the property doesn‘t exists
delete o.x;                // Delete the property x
"x" in o                   // false: it doesn‘t exist anymore