Getting a property of undefined

[littledan]

What is the code triggering the message?

let z = { a: { b: { } } }
z.a.b.c.d

What went wrong in the code? What is the system trying to tell you?

That z.a.b.c is undefined, so you can't access the d property on it

What error message do you see? Which JavaScript environment produced this error?

#### jsc
TypeError: undefined is not an object (evaluating 'z.a.b.c.d')

#### V8
TypeError: Cannot read property 'd' of undefined

#### chakracore
TypeError: Unable to get property 'd' of undefined or null reference

#### spidermonkey
TypeError: z.a.b.c is undefined

eshost shows that only SpiderMonkey gives helpful output

What error message would you like to see?

SpiderMonkey-style output in all JS engines

Do you agree to license your suggested error message under an MIT-style license?

Yes

[codehag]

Playing around with elm style errors again.. just to see what they might look like:

TypeError: z.a.b.c is undefined.

   --> components/some.js:511:26
511 |        z.a.b.c.d
    |              ^^^
    |              |
    |              Property 'c' is undefined. Cannot access property 'd'. 

[brittanydionigi]

Another example related to this...

class Calculator {
  constructor() {
    this.value = 0;
  }

  add(number) {
    this.value += number;
  }

  subtract(number) {
    this.value -= number;
    return this;
  }
};

let cal = new Calculator();
cal.subtract(3).add(5).add(2);

This code, which forgets to return this from the add method, gives you the following error:

TypeError: Cannot read property 'add' of undefined

I really like the Elm style pointers that tell you exactly there something is undefined, but in this case, when a method is returning undefined, it would be nice to be even more explicit and say something like The add() method of cal is returning undefined.

I know that's probably a long shot, but A GIRL CAN DREAM.

[getify]

I feel like the spidermonkey message is incomplete. I read that 4 times before I realized it was only talking about 'c' instead of 'd'. While it's correctly identifying the root problem, it doesn't mention the symptom at all and that seems surprising.

I suggest:

"TypeError: z.a.b.c.d could not be evaluated because z.a.b.c is undefined."