Incorrect error of use of unassigned variable

[NN---]

TypeScript Version: 2.1.5
strictNullChecks: true

Code

let a: {};

if (!a) { // <!-- Error
   ....
}

Expected behavior:
Code compiles fine.

Actual behavior:
Use of unassigned variable.

[devdoomari]

a has the type of {}, and has no value assigned.
i.e. your code is equivalent to:

let a: {} = undefined;

so what you wanted:

let a = {}; // not let a: {}

[NN---]

@devdoomari It is not what I want.
I want to have it undefined and assign it only if it is undefined.

Full code:

let a: {};

for (const x of y) {
  if(x.q) {
    a =x.q;
    break;
  }
}

// Assign default value if couldn't assign before
if (!a) {
  a = {};
}

[ahejlsberg]

The control flow checker (in strict null checks mode) assures that you'll never accidentally observe undefined in a variable unless the type of the variable allows it. Your program logic is constructed such that undefined is actually a permitted value (at least in parts of the logic), so I'd recommend including undefined in the type of a:

let a: {} | undefined;

The problem with your suggestion is that we'd no longer catch this:

let a: string;
// Intended to have assignment here, but forgot
if (!a) {
    // Think I have an empty string, but I have undefined
}

[NN---]

@ahejlsberg What about this case ?

I would expect it compiling fine since I am checking against undefined.

let a: number;
if (typeof a !== "undefined") {
	a = 1;
}

[ahejlsberg]

I think the same argument holds. If the declared type of a variable does not include undefined, the control flow analyzer takes it to mean you never want to observe the variable in an uninitialized state (in any way). I think the consistency of that is important, particularly since you can just add undefined to the type in cases where you want to do those types of checks.

[NN---]

That's what I ended with.
Thanks.

[NN---]

@ahejlsberg
I feel that there is a need for language construct or helper which tests for null or undefined.
Currently I use "if(a) {..}" but if type of 'a' changes to string or number, this test will not be correct.

I feel more safe using this helper now.

function isNullOrUndefined<T>(obj: T | null | undefined): obj is null | undefined {
	return typeof obj === "undefined" || obj === null;
}

if (!isNullOrUndefined(a)) { ... }

But prefer to have something in the language like:

iff (a) { ... }

[ahejlsberg]

Adding entirely new statements beyond what JavaScript has is definitely out of scope for TypeScript. To check if a value is null or undefined in JavaScript, just use x == null. See more discussion here.

[NN---]

Thanks.
Didn't think about this pattern