Check arguments in JavaScript Object Literal Argument

The problem is not with object literals or destructuring — it’s due to prototype inheritance.

All objects in JavaScript already have a toString method via Object.prototype, so this:

function f({ toString, obj1 }) { var existing = toString || obj1; }

will almost always treat toString as truthy, even if it was never passed. That’s why you get a false positive.

Correct approach

You need to check whether the property exists on the object itself, not on its prototype. The modern and concise way is:

function f(arg = {}) { if (Object.hasOwn(arg, "toString")) { return arg.toString; } if (Object.hasOwn(arg, "obj1")) { return arg.obj1; } }

This avoids the verbosity of hasOwnProperty and correctly ignores inherited properties.

Why other attempts fail

Destructuring ({ toString, obj1 }) reads inherited properties too Default values (= null) only apply when the property is undefined, not when it exists on the prototype Spreading ({ ...arg }) does not remove inherited behavior in this case

Recommendation

Avoid using property names like toString in option objects, since they clash with built-in prototype methods. If you rename it (e.g. stringifyFn), your original destructuring approach will work as expected.