typescript forEach expecting additional conditions

[dgofman-equinix]

I cannot understand why in typescript forEach loop the behavior is different comparing with for, for in, or for of loops???

let a: {str: string} | null = null,
    array = ["WORLD", "!"];
if (!a) {
    a = {str: "HELLO"};
    array.forEach(x => {
        a.str += x; //WHY???
        if (a) {
            a.str += x; //OK
        }
        if (a.str) { //HOW???
            a.str += x; //error
        }
        if (a && a.str) { //OK
            a.str += x; //OK
        }
    });

    for (let i = 0; i < array.length; i++) {
        a.str += array[i]; //OK
    }

    for (let i in array) {
        a.str += array[i]; //OK
    }

    for (let i of array) {
        a.str += i; //OK
    }
}

[mhegazy]

Please see #10982 (comment) for an explanation.

consider using const a = { str: "HELLO" }; instead.

[dgofman-equinix]

But my variable is dynamic based on if condition or may be initializing from constructor. I cannot use const the best what can I do use "readonly" (finalized) . But I will get the same issue.

[dgofman-equinix]

Why in my static function I have same issue. I am validating "a" (is not null). Other loops and "return" from a function are not complaining except forEach function.

static concat(a:{str: string} | null, array = ["WORLD", "!"]): string {
        if (!a) {
            a = {str: "HELLO"};
            array.forEach(x => {
                a.str += x; //WHY???
                if (a) {
                    a.str += x; //OK
                }
                if (a.str) { //HOW???
                    a.str += x; //error
                }
                if (a && a.str) { //OK
                    a.str += x; //OK
                }
            });

            for (let i = 0; i < array.length; i++) {
                a.str += array[i]; //OK
            }

            for (let i in array) {
                a.str += array[i]; //OK
            }

            for (let i of array) {
                a.str += i; //OK
            }
        }
        return a.str; //OK
    }

[typescript-bot]

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