block7.ts

function has<P extends string>(obj: object, prop: P): obj is {[K in P]: any} {
    return obj && obj.hasOwnProperty(prop);
}

const testObj: object = null;
if (has(testObj, 'name')) {
	//TypeScript know that testObj has the property name
    console.log(testObj.name); 
    //console.log(testObj.firstname) wouldn't compile
}