`util.isError` uses duck typing

I’m working with Bluebird 3.4.0.

I’ve noticed that util.isError uses various property checks on the error to decide whether the thing you rejected the promise with is an error.

js/release/util.js, lines 225-230:

function isError(obj) {
    return obj !== null &&
           typeof obj === "object" &&
           typeof obj.message === "string" &&
           typeof obj.name === "string";
}

Is there any reason why it doesn’t allow for obj being an instance of Error with alternative property types? The reason I ask is that we use a custom error type that extends Error, and we often use an object as the message, and as things currently are, we get a warning: (node:9482) Warning: a promise was rejected with a non-error: [object Object]

Perhaps the following would be a reasonable change:

function isError(obj) {
    return obj instanceof Error || (
           obj !== null &&
           typeof obj === "object" &&
           typeof obj.message === "string" &&
           typeof obj.name === "string"
    );
}

[ktrain]

@benjamingr In this case, the custom error type being thrown signifies an input/missing data error, not something unexpected that should end up logged in New Relic. It’s used for communication between modules and middleware. If something really crazy were to happen, I’d throw a regular Error.

@spion Yes - that’s going to be my next approach since it looks like the util.isError is written with good purpose. Just wanted to float the idea first and see what the decision was behind its implementation.

Thanks for the great feedback!

[benjamingr]

@ktrain ECMAScript errors have string .message properties - it’s on Error#prototype and in the spec. If I made my own array subclass that defined .length as an object I don’t think it would work in a lot of places.

I don’t think tooling that recognizes and monitors errors (like NewRelic, app insights etc) would recognize your errors.

(Also, my opinion is that stack traces are very important for writing debuggable code, but that’s just my personal opinion)