Suggestion: Let null assertion (!) operator block "used before assigned" errors

TypeScript Version: nightly (2.1.0-dev.20161008)

Code

(Compile with --strictNullChecks)

let n: number
doit(() => {
    n = 0
})
n! + 1

function doit(action) { action() }

Expected behavior:

No error

Actual behavior:

a.ts(5,1): error TS2454: Variable 'n' is used before being assigned.

The problem is fixed by using let n: number | undefined, but it's not obvious that that's the solution, and it would be intuitive for the ! operator to work in this situation.

Aside: I don't see the ! operator documented in the handbook.

Mark Wubben (@novemberborn)

Yes, some solution to immediately invoked callbacks would be great. Mark Wubben (@novemberborn) and others have had issues trying to convert a Promise into a deferral and we have had to actually assign noop functions to values to work around the CFA. For example:

const noop = () => { };

function createDeferral() {
    let complete = noop;
    let cancel = noop;
    const promise = new Promise<void>((resolve, reject) => {
        complete = resolve;
        cancel = reject;
    });

    return { complete, cancel, promise };
}

const deferral = createDeferral();

Which sort of seems silly. I assume we would rewrite it as this with this proposal:

function createDeferral() {
    let complete: () => void;
    let cancel: () => void;
    const promise = new Promise<void>((resolve, reject) => {
        complete = resolve;
        cancel = reject;
    });

    return { complete!, cancel!, promise };
}

const deferral = createDeferral();

I also second the "undocumented" nature of !. Several of the team found it by accident by searching issues.

As I mentioned in #11498 I am more supportive of an annotation that indicates that the function is immediately invoked than this solution.

Approved since "not assigned yet" is really the same thing as "still has undefined in its domain"

fix microsoft#11463, null assertion block uninitialized error

fix #11463, null assertion block uninitialized error (#14020)

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Suggestion: Let null assertion (`!`) operator block "used before assigned" errors #11463

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Suggestion: Let null assertion (!) operator block "used before assigned" errors #11463

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kitsonk commented on Oct 9, 2016

kitsonk commented on Oct 11, 2016

RyanCavanaugh commented on Oct 31, 2016

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Suggestion: Let null assertion (`!`) operator block "used before assigned" errors #11463