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Fix explanation in runtime.onMessage doc #3903

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May 21, 2021
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Fix explanation in runtime.onMessage doc #3903

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30c5ce1
Fix explanation in runtime.onMessage doc
sideshowbarker Apr 7, 2021
074b574
Update in response to review comments
sideshowbarker May 19, 2021
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7 changes: 3 additions & 4 deletions files/en-us/mozilla/add-ons/webextensions/api/runtime/onmessage/index.html
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Original file line number Diff line number Diff line change
Expand Up @@ -111,7 +111,7 @@ <h3 id="Parameters">Parameters</h3>
<p>The <code><var>listener</var></code> function can return either a Boolean or a {{jsxref("Promise")}}.</p>

<div class="notecard note">
<p><strong>Important:</strong> Do not call <code>addListener()</code> using an <code>async</code> function:</p>
<p><strong>Important:</strong> If you pass an async function to <code>addListener()</code>, the listener will return a Promise for every message it receives, preventing other listeners from responding:</p>

<pre class="brush: js example-bad">// don't do this
browser.runtime.onMessage.addListener(
Expand All @@ -121,15 +121,14 @@ <h3 id="Parameters">Parameters</h3>
);
</pre>

<p>This will cause the listener to consume every message it receives, effectively blocking all other listeners from receiving and processing messages.</p>

<p>If you want to take an asynchronous approach, use a Promise instead, like this:</p>
<p>If you only want the listener to respond to messages of a certain type, you must define the listener as a non-<code>async</code> function, and return a Promise only for the messages the listener is meant to respond to — and otherwise return false or undefined:</p>

<pre class="brush: js example-good">browser.runtime.onMessage.addListener(
  (data, sender) =&gt; {
if (data.type === 'handle_me') {
  return Promise.resolve('done');
  }
return false;
}
);
</pre>
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