navigator.clipboard.readText() is not working in Firefox

I'm guessing you are testing it in Firefox, in that case:

Firefox only supports reading the clipboard in browser extensions, using the "clipboardRead" extension permission.

https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/readText#browser_compatibility

Reading clipboard on Firefox (version 94, nov 2021) doesn't seem to work and throws an error:

var promise = navigator.clipboard.readText();

// Uncaught TypeError: navigator.clipboard.readText is not a function

Documentation on MDN Web Docs suggests to grant permission via Permission API:

The "clipboard-read" permission of the Permissions API must be granted before you can read data from the clipboard.

But clipboard-read doesnt seem to be supported:

navigator.permissions.query({ name: "clipboard-read" });

// Uncaught TypeError: 'clipboard-read' is not a valid value for enumeration PermissionName.

The only way to enable clipboard reading (and writing) is to enable dom.events.testing.asyncClipboard on Firefox client:

1. Enter about:config in navigation bar
2. Click "Accept the Risk and Continue"
3. Search dom.events.testing.asyncClipboard and set true