Defining TypeScript variable type function or string

If you try to use the code from seriesOne you will notice, that you cannot assign form (or field) a value that isn't a function.You would get a
Type '{ form: string; }' is not assignable to type 'IParameters'. Types of property 'form' are incompatible. Type 'string' is not assignable to type '() => string'.

I found that using form: (() => string) | string; resolves this problem.

You can try the code at the typescript playground.

Working sample:

    interface IParameters {        form: (() => string) | string;    }    function strFunct(): string {        return 'Hello TypeScript';    }    function test() {        let paramA: IParameters = {            form: strFunct        }        let paramB: IParameters = {            form: 'Hello stackoverflow'        }    }    class Foo {        constructor(param: IParameters) {            var x = typeof param.form === 'string' ? param.form : param.form();        }    }

Perhaps if you define your union type using a call signature rather than Function, like so...

interface IParameters {    // form is of type function that returns number, or number literal.    form: () => number | number;    // field is of type function that returns a string, or string literal.    field: () => string | string;}class Foo {    constructor (param: IParameters) {        var x = typeof param.form === "number" ? param.form : param.form();        var y = typeof param.field === "string" ? param.field : param.field();    }}

Here, I am still checking form and field using typeof, however TypeScript is happy for it to be either a function call, or a value.

Upd
That behavior appears to be an issue #3812 with TypeScript.
It is to be fixed as a part of TypeScript 2.0 according to the milestone assigned.

Original answer

You could use instanceof as a type guard like

var field = (_oParameters.field instanceof Function ? _oParameters.field() : _oParameters.field);