从打字稿类型中删除 | null

Question

我刚开始学习TypeScript，在某些情况下，我得到的可能是Type或null。有没有一种优雅的方式来处理这些案件？

function useHTMLElement(item: HTMLElement) {
console.log("it worked!")
}
let myCanvas = document.getElementById('point_file');
if (myCanvas == null) {
// abort or do something to make it non-null
}
// now I know myCanvas is not null. But the type is still `HTMLElement | null`
// I want to pass it to functions that only accept HTMLElement.
// is there a good way to tell TypeScript that it's not null anymore?
useHTMLElement(myCanvas);

我写了以下似乎有效的函数，但这似乎是一种常见的情况，我想知道语言本身是否为此提供了一些东西。

function ensureNonNull <T> (item: T | null) : T {
if (item == null) {
throw new Error("It's dead Jim!")
}
// cast it
return <T> item;
}
useHTMLElement(ensureNonNull(myCanvas));

Answer 1

如果你实际上在if块中做了一些事情来使myCanvas非null，TypeScript 会识别出：

let myCanvas = document.getElementById('point_fiel');
if (myCanvas == null) {
return; // or throw, etc.
}
useHTMLElement(myCanvas); // OK

或

let myCanvas = document.getElementById('point_fiel');
if (myCanvas == null) {
myCanvas = document.createElement('canvas');
}
useHTMLElement(myCanvas); // OK

Answer 2

Typescript typeguard 还可以识别 instanceof 运算符 - 当 not-null 不是您需要知道的全部内容时很有用

let myCanvas = document.getElementById('point_fiel');
if (myCanvas instanceof HTMLCanvasElement) {
useHTMLElement(myCanvas);
} else if (myCanvas instanceof HTMLElement) {
// was expecting a Canvas but got something else
// take appropriate action 
} else {
// no element found 
}