What is the best way to eliminate a mistaken error in TypeScript (specifically error: TS2339)?

var out = document.getElementsByClassName('myclass')[0];
out.focus();
out.select();
out.selectionStart =1;

I've encountered an issue in my TypeScript file while attempting to execute the above code. Unfortunately, it's throwing errors at me:

ERROR in src/app/main/utilities/keyboard/keyboard.component.ts(29,9): error TS2339: Property 'focus' does not exist on type 'Element'. src/app/main/utilities/keyboard/keyboard.component.ts(30,9): error TS2339: Property 'select' does not exist on type 'Element'. src/app/main/utilities/keyboard/keyboard.component.ts(31,9): error TS2339: Property 'selectionStart' does not exist on type 'Element'.

The compiler is claiming that these properties do not exist even though they should. Despite this, when I execute the code, it functions as expected. However, I'm left with a large block of intimidating red text in my console which is quite bothersome.

angulartypescript

Answer №1

To ensure that Typescript recognizes the methods and properties of your out variable, you must cast it as an HtmlElement.

var out = document.getElementsByClassName('myclass')[0] as HtmlElement;

An alternative approach is:

(out as HtmlElement).focus();

Another option is:

(<HtmlElement>out).focus();

However, keep in mind that you will need to assert the type every time you use the out variable.

For more information on type casting and type assertions, visit: https://www.typescriptlang.org/docs/handbook/basic-types.html

Answer №2

When the compiler encounters a Element and you know it is actually an HTMLInputElement, it may issue a warning. If you are confident in the type of element you have, you can use an assertion as suggested by @vincecampanale. Alternatively, you can employ a user-defined type guard to verify:

function isHTMLInputElement(x: Element): x is HTMLInputElement {
  return (x.tagName.toUpperCase() === 'INPUT');
}

var out = document.getElementsByClassName('myclass')[0];

// ensure that out is indeed an input element
if (!isHTMLInputElement(out)) 
  throw new Error("My life is a lie");

// from now on, the compiler recognizes out as an input element
out.focus(); // all good
out.select(); // all good
out.selectionStart = 1; // all good

The above code satisfies the compiler using control flow analysis to understand that everything after out.focus() will only execute if out is of type HTMLInputElement. This approach eliminates the need for repeated assertions regarding the type of out.

I trust this explanation proves useful.

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