Issue
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.
The html looks as following:
<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>
And the greeter.ts file:
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);
}
When I compile with tsc I get the following “error”:
/home/bjarkef/sandbox/greeter.ts(8,53): The property 'value' does not exist on value of type 'HTMLElement'
However the compiler does output a javascript file, which works just fine in chrome.
How come I get this error? And how can I fix it?
Also, where can I look up which properties are valid on a 'HTMLElement' according to typescript?
Please note I am very new to javascript and typescript, so I might be missing something obvious. 🙂
Solution
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.
So a solution is to cast the result of getElementById() to HTMLInputElement like this:
var inputValue = (<HTMLInputElement>document.getElementById(elementId)).value;
<> is the casting operator in typescript. See the question TypeScript: casting HTMLElement.
If you’re in a .tsx file the casting syntax above will throw an error. You’ll want to use this syntax instead:
(document.getElementById(elementId) as HTMLInputElement).value
The resulting javascript from the line above looks like this:
inputValue = (document.getElementById(elementId)).value;
i.e. containing no type information.
Answered By – Bjarke Freund-Hansen
Answer Checked By – Candace Johnson (BugsFixing Volunteer)