Node: instanceof with arrow function generates error

[jgovaerts]

• Version: v7.10.0
• Platform: x86_64 GNU/Linux
• Subsystem:

Calling instanceof on an object with as argument an arrow function generates an error. I know this particular kind of check is useless, but a library I'm using does this check on functions passed to the library.

I would expect a return value of false as result of this instanceof call with arrow functions.

Example code:

> [] instanceof (()=>{})
TypeError: Function has non-object prototype 'undefined' in instanceof check
    at Function.[Symbol.hasInstance] (<anonymous>)
    at repl:1:4
    at ContextifyScript.Script.runInThisContext (vm.js:23:33)
    at REPLServer.defaultEval (repl.js:339:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.onLine (repl.js:536:10)
    at emitOne (events.js:101:20)
    at REPLServer.emit (events.js:191:7)
    at REPLServer.Interface._onLine (readline.js:241:10)

[goto-bus-stop]

This is not a bug, but spec behaviour. Arrow functions can't have instances, and have no .prototype to check against. You'll have to pass a normal function to the library instead.

[addaleax]

Ack, whether we like it or not, that’s what the spec says. I’m closing this as an answered question, but feel free to ask any follow-up questions that you may have.