Rule proposal: no-valueof-field / no-valueof

[jfmengels]

Rule proposal: Forbid specifying a valueOf field in an object.

const object = {
  valueOf: () => 2 // or do something odd
};
object+''
// => '2'

Having valueOf overriden for an object, or any value, can cause some unexpected, or simply implicit behavior.

Invalid

const object1 = {
  value: 15,
  valueOf: function() { return this.value; }
};
const object2 = {
  value: 25,
  valueOf: function() { return this.value; }
};
object1 + object2
// => 40

Valid

const addValueOf = (a, b) => a.value + b.value;

const object1 = {
  value: 15
};
const object2 = {
  value: 25
};

addValueOf(object1, object2)
// => 40

I'm wondering whether this should also check for X.prototype.valueOf = ..., X.valueOf = .... I'm not sure as it's already kind of covered by no-mutation, and I'd prefer not to create 2 errors for it. I still think it should (and then the name should be changed, probably to no-valueof) as people can turn on this rule and not no-mutation.

And that brings another question, should no-valueof then also forbid calling valueOf explicitly?

If people are watching, would gladly get some feedback on this :)

[jfmengels]

Decided not to forbid calling valueOf explicitly in this rule. Let me know if someone thinks that should be a new rule.

[jfmengels]

@bodil This rule was just added in v2.2.0. Just a heads up in case you'd like to remove it from cleanjs.

[bodil]

No, I like it. :)