instanceof is accepting a string as an instance of my Type class Javascript

Question

The if statement below evaluates to true when type is a string and I can't seem to figure out why. This is my code:

const validateType = (instanceDescription, type) => {
    if (!type instanceof Type) {
        throw new Error(`type property of ${instanceDescription} is not 
                         a (child) instance of class Type`);
    }
}

I can't see the problem being in my class because it's really simple. It looks like this.

class Type {
    constructor(key, multipliers) {
        this.multipliers = multipliers;
        this.key = key;
    }
}

Is there something going on with the instanceof comparison that I'm not aware of or am I just going mad. I got around it by checking if a certain property is undefined which it will be for a string but I would rather go for the more clear instanceof option

and more precisely in which language? there is not tag to mark the language. — User007
– User007, Commented Sep 23, 2018 at 20:16
Could you add how you are calling the validateType function, and with what parameters? — Icepickle
– Icepickle, Commented Sep 24, 2018 at 7:41

Mark · Accepted Answer · 2018-09-24 07:47:45Z

2

Parenthesis make a difference here because of operator precendence. ! has a higher precedence than instanceof so with out the parenthesis your test is asking if false is an instance of Type:

class Type {
  constructor(key, multipliers) {
      this.multipliers = multipliers;
      this.key = key;
  }
}

let t = "somestring"

if (!(t instanceof Type)) { // << note the parenthesis
  console.log("error")
}
if (!t instanceof Type) {  // << never fires
  console.log("no error")
}

answered Sep 24, 2018 at 7:47

Mark

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