Is there a way to force "if" to only accept boolean in TypeScript?
The following code is legal in TypeScript:
let asyncCondition = async(): Promise<boolean> => // .. calculate & return a boolean
if(asyncCondition()){ // forgot to await. condition is always true
// .. do stuff
}
Since asyncCondition() returns a normal not-null Promise , the code in if block will always get executed. This is JavaScript behavior and it's understandable that TypeScript doesn't complain.
But in the above scenario, what I really meant is:
let asyncCondition = async(): Promise<boolean> => // .. calculate & return a boolean
if(await asyncCondition()){ // condition is asyncCondition()
// .. do stuff
}
Is there a way to let TypeScript to type check this kind of errors for me?
The compiler does not do it and I won't expect it to do it any time soon. It has been asked and rejected a number of times. The cases I could find:
In each cases, the reasoning for closing these issues without changing the compiler was that it would be too radical a change, and that really a linter should do this job.
The good news is that a new rule has recently been merged into tslint 's codebase to provide warnings about this problem. As far as I can tell, though, the rule is not yet in a released version of tslint . Once it is released, if you set strict-boolean-expressions to true in your tslint rules, then tslint will warn you when you use a conditional with an expression that is not strictly boolean.