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情况?
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);
我编写了以下似乎有效的函数,但这似乎是一种常见情况,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));
最佳答案
如果您实际上在if
block 中做一些事情来使myCanvas
非null
,TypeScript 将识别那:
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
关于javascript - 删除 |来自 Typescript 类型的 null,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45427155/