Failed check on function in if clause

[huhuanming]

TypeScript Version: 2.5.3

Code

if (Math.floor(1.2), 2) {
   // do something
}

Expected behavior:

error TS2695: Left side of comma operator is unused and has no side effects.

Actual behavior:

Nothing happened.

But can get error like this.

[kitsonk]

There are limits to things like call flow analysis, and this would be one of them. While it might be potentially a logic error, what is a real world use case where this would occur. CFA comes at a cost, and if this is not a common logic error, it is unlikely for it to be added to the compiler.

[huhuanming]

But I really did take a very long time to find this error in real world.

As you can see, I just made a example.

[huhuanming]

And Why if (1, 2 is error, but if (Math.floor(1.2), 2) can pass.

[kitsonk]

A simplified example is always good, but again, that is not very realistic or sensible code. If you provided an example that was more realistic, then it might help identify how TypeScript could help in statically identifying issues.

There are trade-offs with CFA (see #9998). This would require the ability to describe the return types of functions in truthy and un-truthy values to be evaluated at run time to produce the same error. While Math.floor() is a built in function, the way TypeScript deals with it is like any other return, which is that it returns a number and when it does the analysis it cannot determine if that number will be 0 (falsey) or != 0 (truthy).

On the other hand, providing more than one statement to an if () expression is likely a static error that TypeScript could detect... I am not really sure any construct like if (foo, bar) { } would ever be desirable, even if it is valid syntax.

This is unrelated to if.

Math.floor(1.2), 2; // No error
1, 2; // Error

Note that the error message says "and has no side effects". TypeScript doesn't track what functions have side effects, so this error will only happen for an expression that obviously has no side effects such as null -- never for a function call.
You may be interested in palantir/tslint#3250.

[huhuanming]

Thx，I got it.
😄

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.