Type guard does not affect variable inside a function

[srolel]

TypeScript Version: 2.7.0-dev.201xxxxx

Code

function foo(o: number | number[]) {
    if (!Array.isArray(o)) {
        o = [o];
    }
    o.length;
    () => o.length;
}

Expected behavior:
No compile errors.

Actual behavior:

Property 'length' does not exist on type 'number'.

in () => o.length

[sylvanaar]

Note that it occurs with this code too:

function foo(o: number | number[]) {
	if (!Array.isArray(o)) {
		o = [o];
	} else {
		o.length;
		() => o.length; // error: Property 'length' does not exist on type 'number'.
	}
}

[SalathielGenese]

I think the TypeScript compiler is using the same scope rules for lambdas and functions. This' the only logical reason I see for this. I think that have to do with Function Statement and Function Expression scope.

A function statement should behave that way because hoisting will make it available in foo top level, not like function expression (and lamdas are always).

Meanwile, though I still can't phatom why @mosho1 wrote it that way, I found that the following works great.

(o => o.length)(o);

[srolel]

@SalathielGenese it's a distilled demonstration of the issue, not my use-case. Your solution does not solve the problem because you are assuming the function needs to be called immediately. It may be passed to another function like Array.prototype.map.

Yes, this should be the case for function statements, but not for function expressions and arrow functions.

[SalathielGenese]

@mosho1, I attempted to :

1. Explain the reason of the issue,
2. Provide a workaround

Another workaround could be :

(o: number[]) => o.length

Still another :

() => (<number[]>o).length

All are work around to get the job done while the issue is being fixed.

[ahejlsberg]

This is working as intended. Type guards remain in effect across function boundaries only when the guarded variable(s) are considered constant. This is the case for const local variables and parameters that never occur as assignment targets in the function. See #10357.

[srolel]

I see. That makes sense, thanks!