javascript - 删除 |来自 Typescript 类型的 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));

Answer 1

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