通过扩展对象文字来实例化Typescript变量

Question

Let's say I have two interfaces by which I want to construct an instance out of. One of the interfaces extends the other.

interface IScope {/*... */}

interface IDialogScope extends IScope { a? : string , b? : string }

Let's say that there is a method in a third party module that let's me instantiate a variable of type IScope

var scope : IDialogScope = ScopeBuilder.build(); /* build actually returns an IScope type */

Now I can populate the scope variable

scope.a = "hello"; scope.b = "world";

If I were using lodash/underscore, I could have just extended an existing object literal with my custom properties and gotten my final object. The problem with TypeScript here is that I can't just create a DialogScope class that implements IDialogScope because then I would also have to implement everything in IScope which I can't because it's from a third party library.

I want to be able to do this in TypeScript:

var scope : IDialogScope = _.extend({}, ScopeBuilder.build(), {a: "hello", b: "world"});

Answer 1

I want to be able to do this in TypeScript:

This is exactly what intersection types are for.

function extend<T, U>(first: T, second: U): T & U {
    let result = <T & U> {};
    for (let id in first) {
        result[id] = first[id];
    }
    for (let id in second) {
        if (!result.hasOwnProperty(id)) {
            result[id] = second[id];
        }
    }
    return result;
}

var x = extend({ a: "hello" }, { b: 42 });
var s = x.a;
var n = x.b;

These were only released recently : https://github.com/Microsoft/TypeScript/pull/3622

It will be a part of TypeScript 1.6.

If you want to use it today you can use ntypescript : https://github.com/basarat/ntypescript