类型断言
有时你会得到关于 TypeScript 无法知道的值类型的信息。
Sometimes you will have information about the type of a value that TypeScript can't know about.
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const myCanvas = document.getElementById("main_canvas") as HTMLCanvasElement;
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const myCanvas = <HTMLCanvasElement>document.getElementById("main_canvas");
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const x = "hello" as number;
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declare const expr: any;
type T = { a: 1; b: 2; c: 3 };
const a = (expr as any) as T;