Add option for no-unused-expressions

I’m using Chai’s expect syntax for testing, and I’m getting no-unused-expression errors. I understand that this is because expressions like expect(...).to.be.undefined look like property access, even though chai’s actually using the property’s getter to call a method behind the scenes that causes some side effects.

I did a bit of googling and at least a few other people have encountered this.

I’m not sure what the best solution is (I don’t know enough about parsing JS to say) but it seems like one possibility might be an option that would not flag property access chains that start with certain identifiers. So the user could specify "expect" as the option’s value and then all these chai calls would be let through.

[AMorgaut]

@nzakas with all respect, you said:

Sorry all, we still don’t have a way of resolving this without baking-in knowledge of Chai. As a general principle, we do not put library-specific code in ESLint.

But actually, ESLint, as JSHint, does support “environment options” such as jasmine, mocha, and qunit, plus protractor, atomtest, and embertest on ESLint side… so I think that adding a chai environment option would not break existing ESLint library approach.

I just wrote a related feature request to the JSHint project

As I described in that feature request, another non ‘library related’ approach could be to add a expr: 'accessor' sub-option to not raise the warning when we ends an expression with the access to an accessor property(cf ES5 Property Attributes) . I mentioned related use cases like forced singleton instantiation.

This library agnostic option can be interesting but:

• it would probably be more complicated to support than just supporting the chai use case
• it could hide expected warnings if we only want it on the chai API (such sub option is probably more legitimate to be activated at line level than at file level)

[niftylettuce]

👍 for dirty-chai

npm install --save-dev dirty-chai

var chai = require('chai');
+// Load dirty chai first to hook plugin extensions
+var dirtyChai = require('dirty-chai');
+chai.use(dirtyChai);