打字稿对象奇怪的类型语法

Question

When reading TypeScript handbook , I came across following example:

interface Shape {
    color: string;
}

interface Square extends Shape {
    sideLength: number;
}

var square = <Square>{};
square.color = "blue";
square.sideLength = 10;

The question is - what is actually <Square>{} ? Seems like a bizarre syntax to me. From Java/C# point of view, it's like a generic of an anonymous object. What exactly is it and what are the limitations of such creation?

Answer 1

It's "casting". Interpret the following thing ( {} , an object literal with no fields) as a Square basically. So because of the usage of that the square will be inferred to be of type Square by the TypeScript compiler and Intellisense will show the correct members.

Of course it's not real "casting", as we know types are just an illusion in TypeScript. It's all for the compiler.

Answer 2

Its called a Type Assertion https://basarat.gitbooks.io/typescript/content/docs/types/type-assertion.html

And the pattern you are looking at:

var square = <Square>{};
square.color = "blue";
square.sideLength = 10;

Is common (but not recommended) for JS -> TS Migration for lazy object initialization : https://basarat.gitbooks.io/typescript/content/docs/tips/lazyObjectLiteralInitialization.html