Union parameters with TypeScript 1.4 - typescript1.4

Question

I have some troubles using union parameters. Imagine I have such a function:
interface IColumnDefinition {
id: string;
title: string;
}
addColumn(definition: string|IColumnDefinition) {
if (typeof (definition) === "string") {
definition = { id: definition, title: definition };
}
var column = new JSColumn(definition);
...
}
I want the user to either pass a complete object that defines a new column, or just pass a string, in which case I create a default object.
However, TypeScript has 2 problems:
definition = { id: definition }; is not allowed, since TypeScript thinks that definition might not be a string - which is wrong, since I make a typeof one line above.
new JSColumn(definition) is not possible, since the constructor requires a IColumnDefinition object, and definition could also be a string - which is also wrong, because the if above made sure that it is always a correct object.
What can I do to convince TypeScript that these are not errors? Is there anything like a cast, which says "Hey, this variable is of type xyz. Trust me, I know what I do."
I'm currently defining the parameter as any, which isn't really an option as I lose the whole advantage of type-checking.

Answer 1

If you want to circumvent the type system of TypeScript you can always use <any> cast.
In this case however there's a better solution:
function addColumn(definition: string | IColumnDefinition) {
var colDef: IColumnDefinition;
if (typeof definition === "string") {
colDef = { id: definition, title: definition };
} else {
colDef = definition;
}
var column = new JSColumn(colDef);
// ...
}
Another option, which might be less clear but generates a smaller JS code:
function addColumn(definition: string | IColumnDefinition) {
var column = new JSColumn(
(typeof definition === "string")
? { id: definition, title: definition }
: definition);
// ...
}