Defining a function with parameters and properties in TypeScript

Question

I've got a function that looks like this:

var myFunction = function (config) {
  var example = this.property; // just illustrating that we use `this`
}
myFunction.__reference = 'foobar';

Now I'm trying to write it in strict TypeScript:

interface ExternalScope {
  property: string;
}

interface ConfigObject {
  name: string,
  count: number
}

interface MyFunction {
  (XHRLoader: this, cfg: ConfigObject): any;
  __reference: string;
}

var myFunction = function (this: ExternalScope, config: ConfigObject): any {
  var example = this.property;
}
myFunction.__reference = 'foobar';

With the above code I get the following TypeScipt error:

Property '__reference' does not exist on type '(this: ExternalScope: config: ConfigObject) => any

The relevant portion of my tsconfig.json :

"compilerOptions": {
    "noEmitOnError": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "strictNullChecks": true,
    "noFallthroughCasesInSwitch": true,
    "moduleResolution": "node",
    "outDir": "./build",
    "allowJs": false,
    "target": "es5"
},

Answer 1

Perhaps this can be of help:

interface ExternalScope {
    property: string;
}

interface ConfigObject {
    name?: string,
    count?: number
}

interface MyFunction {
    (XHRLoader: this, cfg: ConfigObject): any;
    __reference?: string;
}

var myFunction: MyFunction = function (this: ExternalScope, config: ConfigObject): any {
    var example = this.property;
}

myFunction.__reference = 'foobar';

Even though myFunction: myFunction creates more errors you need to assign it.

when you simply do

var myFunction = function (this: ExternalScope, config: ConfigObject):  any {
    var example = this.property;
}

typescript tries to infer the type of the myFunction variable and since it cannot see any __reference property in the assigned function, the inferred type does not contain it either. Hope it helps :)