Union Type inference not working in class (ts version 1.4)

Hi, Union type inference is not working inside classes functions. For example trying to compile the below code :

type NameOrNameArray = string | string[];

class NameCreator {

    constructor(public name:NameOrNameArray) {}

    createName():string {
        if (typeof this.name === "string") {
            return name;
        }
        else {
            this.name.forEach((elem)=>{
                console.log(elem);
            });
            return this.name.join(" ");
        }
    }
}

gives the error :

Property 'forEach' does not exist on type 'string | string[]'.

The same code works well when using a function. Same error in Visual studio and in the Playground.

[ahejlsberg]

Type guards only work on simple variables, not properties of an object, so you need to copy the property into a local variable.

type NameOrNameArray = string | string[];

class NameCreator {

    constructor(public name: NameOrNameArray) {}

    createName(): string {
        var name = this.name;  // Copy into variable
        if (typeof name === "string") {
            return name;
        }
        else {
            name.forEach(elem => {
                console.log(elem);
            });
            return name.join(" ");
        }
    }
}

We have this restriction because we want to track (to the best of our abilities) that there are no assignments to the variable in the guarded section. This is pretty much impossible to do correctly for properties without extensive flow and alias analysis. For example:

function printString(s: string) { ... }
class Foo {
    name: string | string[];
    foo() {
        if (typeof this.name === "string") {
            this.bar();
            printString(this.name);  // Ok?
        }
    }
    bar() {
        this.name = ["hello"];
    }
}

This is just a simple example. You can construct increasingly impossible ones with little effort.

A possible alternative to our current design is to allow type guards to work on dotted names of the form x.y.z or this.x.y.z and to ignore the effects of assignments to such names (or parts of them). But it would mean that we wouldn’t catch errors like the one in the example above.

[erichillah]

I am able to remove the error only by using explicit type assertion

(<string[]>this.name).forEach((elem)=>{
    console.log(elem);
 });
 return (<string[]>this.name).join(" ");