Is there a way to force "if" to only accept boolean in TypeScript?

Question

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?

Answer 1

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:

• Feb 2016: https://github.com/Microsoft/TypeScript/issues/7306
• Apr 2016: https://github.com/Microsoft/TypeScript/issues/8178
• Jul 2016: https://github.com/Microsoft/TypeScript/issues/9702

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.

Answer 2

I came across a similar issue and found the strict-boolean-expressions rule. Not the perfect answer for the example given by the OP, but might be helpful for others.

Activating this rule causes expressions like

if (nonBooleanCondition) {

to display the following warning:

This type is not allowed in the 'if' condition because it is a [TYPE]. Only booleans are allowed.