Rule falsely claims dissimilar types when they might match

[unblocker (Ben Towne)]

Rule javascript:S3403 says:
Comparing dissimilar types using the strict equality operators === and !== will always return the same value, respectively false and true , because no type conversion is done before the comparison. Thus, such comparisons can only be bugs.

Sonarqube version: Version 8.9.5 (build 50698)

In the below example, Sonarqube incorrectly determines the type of variable ‘alsoDoY’ to be only a boolean, because that is what it is initially assigned to. However, the variable can change type to String ‘cancel’ conditionally, upon the condition needsUserInput and the user selecting “cancel”.

let askQuestion = function(question, yes, no, cancel) {
	//Updates the UI with the question,
	//calls one of the the callback functions according to what the user selects.
}

let getUserInput = function(question) {
	return new Promise(function(resolve,reject) {
		askQuestion(question, function yes() {
			resolve(true) ;
		}, function no() {
			resolve(false) ;
		}, function cancel() {
			resolve('cancel') ;
		});
	})
}

let doY = function() {
	//...
}

let doX = async function(needsUserInput) {
	let alsoDoY = false;
	if(needsUserInput) {
		alsoDoY = await getUserInput('You have requested X. Do you want to do Y as well?');
	}
	if(alsoDoY === 'cancel') {
		throw(new Error('User cancelled operation.'));
	}
	//processing for X
	if(alsoDoY) {
		doY();
	}
}

Javascript is not a strictly typed language, and Sonarqube errs by treating it that way.

[Lena (Elena Vilchik)]

Hi Ben,

Thanks for reporting this, I created the ticket to track it FP S3403: in JavaScript type of the initializer is considered · Issue #2925 · SonarSource/SonarJS · GitHub.

[unblocker (Ben Towne)]

Thank you for opening the ticket and simplifying the example.