Ah, I've run into this exact issue with Twitch's chat before! The problem here is that Twitch uses a frontend framework (likely React) to manage the chat input. When you directly set the value property on the textarea element like you did, you're only modifying the visible DOM—but the framework maintains its own internal state that drives what gets rendered. That's why the "source code" (the framework-managed state reflection in the DOM) doesn't update, and if you tried to send the message, it might not actually send the text you set.

Here's how to fix it:

• Use a more reliable selector (instead of relying on classes that might change): Twitch uses data attributes for targeting elements, so [data-a-target="chat-input"] is far more stable than the class string.
• Trigger the right event to tell the framework that the input has changed. Frontend frameworks like React listen to input events to sync their internal state with the DOM value.

Working Code Example #

// Grab the chat input element using the stable data attribute
const chatInput = document.querySelector('[data-a-target="chat-input"]');

if (chatInput) {
  // Set your desired message text
  chatInput.value = "Your message here!";
  
  // Create and dispatch an input event to sync with the framework's state
  const inputEvent = new Event('input', {
    bubbles: true, // Ensures the event propagates like a real user input
    cancelable: true
  });
  
  chatInput.dispatchEvent(inputEvent);
}

Why This Works #

When you dispatch the input event, the framework picks it up just like it would if you typed the text manually. It reads the current value of the textarea, updates its internal state, and then re-renders the element to match—so now the DOM's visible content, the framework's state, and even the "source code" view of the element will all be in sync.

If you also want to automate sending the message after setting it, you can trigger the send button's click event too:

// Grab the send button (again using the data attribute)
const sendButton = document.querySelector('[data-a-target="chat-send-button"]');
if (sendButton) {
  sendButton.click();
}

内容的提问来源于stack exchange，提问作者deathnoob