Detecting nested undefined attributes using command line tools (eslint/standard)

Question

I'd like to be able to use static code analysis tool (standard/eslint) to detect following situation:

  const obj = {a: {b: 'just a value'}}
  // should be obj.a.b
  const b = obj.a.c
  // so b will be undefined

Both Standard and ESLint will not find any issue here. Is it possible to detect it using good code quality tools?

Just to give example, IDEA/Webstorm is reporting the problem correctly

so, just wondering, is it possible to detect the same issue using command line tools.

Answer 1

Static type checkers like Flow or TypeScript are designed to catch this class of bugs. Try pasting your example code in either the try Flow editor or the TypeScript playground . Without any modifications to the code to add specific type annotations, both checkers catch the error.

Flow :

4: const b = obj.a.c
                   ^ property `c`. Property not found in
4: const b = obj.a.c
             ^ object literal

TypeScript :

Property 'c' does not exist on type '{ b: string; }'.

Answer 2

In JavaScript, undefined is a correct value for variables, so you can't detect an assignment of this value. For your problem, I suggest you to define a function like this one :

const assignNested = function(value) {
  if (value === undefined)
    throw new Error("undefined assignment");
  return value;
}

Then you can rewrite your code as followed :

const obj = {a: {b: 'just a value'}};
const b = assignNested(obj.a.b); // b = 'just a value'
const c = assignNested(obj.a.c); // raise exception

Don't forget to catch your throwed exceptions. If you want define your own exception type, it's possible (take a look at https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/Error ).

Answer 3

Javascript is not a typed language, so it's not really possible to validate object destructuring before running your code.

For example, when you get a response from an HTTP Request, your code quality tools are not aware of the resulting JSON object.