Logial operator in a variable assignment isn't defaulting to the always true statement

Question

I don't understand why this doesn't work when the first value isn't available:

var borough = state.city.borough || 'Manhattan';

It returns:

Uncaught ReferenceError: state is not defined

I would expect it to default to Manhattan when state.city.borough isn't defined.

Edit: I am looking for a solution that will work if the "state" object or any of it's keys are not available.

You can't dereference an object that is not defined, so you have to check for all potentially undefined parts: `var borough = state && state.city && state.city.borough || 'Manhattan';` — Hunter McMillen, Mar 29 '18 at 18:17
If `state` is not declared/defined your code won't work at all — Ele, Mar 29 '18 at 18:17
Possible duplicate of [Test for existence of nested JavaScript object key](https://stackoverflow.com/questions/2631001/test-for-existence-of-nested-javascript-object-key) — jmargolisvt, Mar 29 '18 at 18:18
@user3006927 Obviously, the variable `state` has to be declared somewhere. Having a value of undefined is not the same as not existing. — Hunter McMillen, Mar 29 '18 at 18:21
It seems like they actually want something like this: `var borough; try { borough = state.city.borough; } catch (e) { borough = 'Manhatten'; }; `. Which is absurd — Hunter McMillen, Mar 29 '18 at 18:25
@HunterMcMillen this is what I'm looking for...Why is it absurd? — user3006927, Mar 29 '18 at 18:47
I can't think of many reasons why you would be trying to reference an object called `state` but have not previously declared it somewhere. Maybe if you showed more of your surrounding code we could help more — Hunter McMillen, Mar 29 '18 at 18:50

skyboyer · Answer 1 · 2018-03-29T20:22:34.300

Once you fixed the reason why state is declared nowhere you may run into another issue: once state or state.city return null or undefined you will still get an error.

Workarounds are:

check every single member in the chain:

var borough = (state && 
            state.city && 
            state.city.borough) || 'Manhattan';

use something similar to lodash's _.get or write your own version:
```
var borough = _.get(state, 'city.borough', 'Manhattan');
```

use destructuring assignment from ES6

 var {city: {borough = 'Manhattan'}} = state;

wait until elvis operator is aprooved as part of standard and implemented in major borwsers(or you may apply Babel to use this right now)
```
var borough = state?.city?.borough || 'Manhattan';
```

[UPD] missed that point: you have syntax error because state has not been ever defined; not because it's null or something like that

I think you need a `this` someplace or you have the same undefined `state` problem in 1 — JonSG, Mar 29 '18 at 19:30
@JonSG I agree that those alternatives will not help in case author does not put declaration for `var state` anywhere. — skyboyer, Mar 29 '18 at 20:24

JonSG · Answer 2 · 2018-03-29T19:40:32.680

0

If state is not declared then it is not declared . However, this is defined so assuming that it is reasonable that at some point to expect state === this.state then try:

var borough = (
    this.state &&
    this.state.city &&
    this.state.city.borough) || 'Manhattan';

edited Mar 29 '18 at 19:40

answered Mar 29 '18 at 19:35

JonSG

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score -1 · Answer 3 · answered Mar 29 '18 at 18:18

-1

This appens because in javascript,if you try to access the property of an undefined object the compiler crashes,so you need to check for the existence of each one of the property you are trying to access:

var borough;

if (state && state.city && state.city.borough) {
  borough = state.city.borough;
}
else {
  borough = 'Manhattan';
}

answered Mar 29 '18 at 18:18

Alessandro Messori

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did you declare the state object somewhere? I assumed that in my answer but if you didn't declare at all the state object of course it will throw an error – Alessandro Messori Mar 29 '18 at 18:23

score -2 · Answer 4 · answered Mar 29 '18 at 18:18

-2

You can't de-reference an object that is no value, so you have to check for all potentially undefined parts:

var state = undefined;
var borough = state && state.city && state.city.borough || 'Manhattan';

answered Mar 29 '18 at 18:18

Hunter McMillen

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Logial operator in a variable assignment isn't defaulting to the always true statement

4 Answers4