Support direct typing when using instanceof for better intellisense

[Alphapage]

Hello,

Here is my code:

let value: any
 if (value instanceof MyClass) {
     // value is any. needs to typecast like this to get intellisense working:
     (<MyClass>value).prop=true;
}

It would be great to support this feature.

Thanks in advance and I hope this could help others.

[Arnavion]

instanceof already narrows types.

let value2: MyClass | string;
if (value2 instanceof MyClass) {
    value2.prop = true; // value2 is a MyClass inside the if-block. No typecast needed.
}

The reason it doesn't in your example is specifically because value is of any type, which instanceof doesn't narrow. See #1426 for why it works this way.

[DanielRosenwasser]

Duplicate of #1595, #1938, and #6301.