Great list - lots of lovely little tidbits, especially the isRequired helper function. Love that! You have to be slightly careful with the default or in 5, though. If age is 0, it will overwrite it (making your new born person object a 35 year old 😉). A safer, albeit slightly less-well-supported (IE doesn't know about it), way of attacking it is to use the nullish coalescing operator - ?? which returns truthy values such as 0. More info available on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator