“HTMLElement"类型的值上不存在属性“value" [英] The property 'value' does not exist on value of type 'HTMLElement'
问题描述
我正在玩打字稿，并试图创建一个脚本，该脚本将在输入框中输入文本时更新 p 元素.
I am playing around with typescript and am trying to create a script that will update a p-element as text is inputted in a input box.
html 如下所示:
<html>
<head>
</head>
<body>
<p id="greet"></p>
<form>
<input id="name" type="text" name="name" value="" onkeyup="greet('name')" />
</form>
</body>
<script src="greeter.js"></script>
</html>
还有 greeter.ts 文件:
function greeter(person)
{
return "Hello, " + person;
}
function greet(elementId)
{
var inputValue = document.getElementById(elementId).value;
if (inputValue.trim() == "")
inputValue = "World";
document.getElementById("greet").innerText = greeter(inputValue);
}
当我使用 tsc 进行编译时，出现以下错误":
When I compile with tsc I get the following "error":
/home/bjarkef/sandbox/greeter.ts(8,53): 'HTMLElement' 类型的值不存在属性 'value'
然而，编译器确实输出了一个 javascript 文件，它在 chrome 中工作得很好.
However the compiler does output a javascript file, which works just fine in chrome.
我怎么会收到这个错误?我该如何解决?
此外，根据打字稿，我可以在哪里查找 'HTMLElement' 上哪些属性有效?
Also, where can I look up which properties are valid on a 'HTMLElement' according to typescript?
请注意，我对 javascript 和 typescript 非常陌生，所以我可能会遗漏一些明显的东西.:)
Please note I am very new to javascript and typescript, so I might be missing something obvious. :)
推荐答案
根据 Tomasz Nurkiewicz 的回答，问题"是 typescript 是类型安全的.:) 所以 document.getElementById() 返回不包含 value 属性的 HTMLElement 类型.但是子类型 HTMLInputElement 确实包含 value 属性.
Based on Tomasz Nurkiewiczs answer, the "problem" is that typescript is typesafe. :) So the document.getElementById() returns the type HTMLElement which does not contain a value property. The subtype HTMLInputElement does however contain the value property.
所以一个解决方案是将 getElementById() 的结果转换为 HTMLInputElement 像这样:
So a solution is to cast the result of getElementById() to HTMLInputElement like this:
var inputValue = (<HTMLInputElement>document.getElementById(elementId)).value;
<> 是打字稿中的转换运算符.请参阅问题 TypeScript:转换 HTMLElement.
<> is the casting operator in typescript. See the question TypeScript: casting HTMLElement.
上一行生成的 javascript 如下所示:
The resulting javascript from the line above looks like this:
inputValue = (document.getElementById(elementId)).value;
即不包含类型信息.
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