TypeError: obj.hasOwnProperty is not a function

[sgoll]

In certain situations, compare fails to calculate a valid JSON patch and throws a TypeError with this message instead:

obj.hasOwnProperty is not a function

In my case, this happens when running objects created by GraphQL's graphql method through fast-json-patch. One location where this error originates is this line in json-patch-duplex.js:

if (obj.hasOwnProperty(key) && !(obj[key] === undefined && oldVal !== undefined && Array.isArray(obj) === false)) {

In fact, this error will be thrown whenever custom objects are used that do not have Object in their prototype hierarchy, such as plain maps created with Object.create(null).

I propose to replace all calls to special Object methods with Object.prototype calls such as this:

// Old:
obj.hasOwnProperty(key)
// Replace with:
Object.prototype.hasOwnProperty.call(obj, key)

This ensures that no exception is thrown in case objects without the Object prototype are used.

One may certainly argue that this seems like a pretty far-fetched corner case, but e.g. ESLint has a special rule against this type of Object prototype builtins: Disallow use of Object.prototypes builtins directly (no-prototype-builtins), see their rationale.

I would be willing to prepare a pull request for the required changes.

[alshakero]

Closing as done and released! Thank you @sgoll 🎉