To save a file in a user’s Downloads folder, use chrome.downloads. To retain an extension’s own settings or data, use chrome.storage. These APIs solve different problems, and mixing them up is a frequent source of Manifest V3 bugs.
The eight checks below are documented failure modes to investigate—not a record of eight experimentally tested incidents. The exact fix depends on whether you are exporting a file for the user or preserving state for the extension.
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First decide what “save extension data” means
A download creates a file in the browser’s configured Downloads directory. Extension storage keeps keyed, JSON-serializable data for the extension to retrieve later. Saving a setting with chrome.storage does not create a file, and calling chrome.downloads is not a substitute for persisting internal state.
- Export a file for the user: use
chrome.downloads. - Keep extension state between events or sessions: use
chrome.storage, choosing its area to match the needed lifetime and sync behavior.
Eight Manifest V3 saving problems to check
1. The manifest does not declare the Downloads permission
The Downloads API cannot be used unless the extension declares "downloads" in its manifest. Chrome’s API documentation states: “You must declare the "downloads" permission in the extension manifest to use this API.” See the Chrome Downloads API reference.
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{
"manifest_version": 3,
"permissions": ["downloads"]
}Request only permissions the extension needs, and explain why file downloads are part of its functionality. A permission may trigger a user-facing warning.
2. The filename is treated as an absolute path
The filename option is a path relative to the user’s Downloads directory, not an arbitrary absolute filesystem path. Absolute paths, an empty path, and paths containing .. are rejected. Use a safe relative subdirectory when you need to organize exports.
chrome.downloads.download({
url: "https://example.com/export.json",
filename: "MyExtension/export.json"
});This writes beneath Downloads; it does not let an extension choose any location on the user’s filesystem.
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3. A suggested filename is assumed to be final
Pass filename to chrome.downloads.download() when you already know the desired name. If the name or MIME type depends on the server response or tentative filename, use chrome.downloads.onDeterminingFilename to suggest one. Each extension can register one listener, and every listener must call suggest() exactly once. If the listener handles that call asynchronously, it must return true so the browser waits for the suggestion.
4. Another extension’s filename listener changes the result
A download can wait for all registered filename listeners to call suggest(). If multiple extensions supply overrides, the last installed extension whose listener provides a suggestion wins. Your extension therefore cannot promise deterministic control over a filename when another installed extension participates.
5. Existing files are handled by an accidental default
Choose conflictAction deliberately when a file with the same name may already exist. The documented choices are:
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uniquify: add a counter before the file extension so the new download gets a distinct name.overwrite: replace the existing file.prompt: ask the user what to do.
For example, a download with an explicit non-destructive collision policy can be started like this:
chrome.downloads.download({
url: "https://example.com/export.json",
filename: "MyExtension/export.json",
conflictAction: "uniquify"
});6. Important state exists only in service-worker globals
Manifest V3 extension service workers are not persistent background pages. Chrome may terminate one after 30 seconds of inactivity; when it shuts down, its global variables disappear. Do not rely on a global variable to hold a download ID, pending job, or other value that a later event needs. Persist the required state with chrome.storage and load it when handling the later event. See the service worker lifecycle documentation.
7. Web Storage is used for extension state in the wrong context
An extension service worker cannot access window.localStorage. In a content script, Web Storage calls address the host page’s storage rather than a dedicated extension store. Use the asynchronous chrome.storage API for extension-owned state; it is available in extension contexts including service workers and content scripts.
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8. Storage lifetime is wrong, or a download is mistaken for a saved state
A completed download only tells you about the file operation; it does not confirm that extension state was persisted. Choose a storage area for the state’s required lifetime, and await asynchronous writes before depending on them.
| Storage area | Lifetime and behavior | Best fit |
|---|---|---|
storage.local |
Persists across browser cache/history clearing, but is cleared when the extension is removed. | Extension data that should remain available locally across sessions. |
storage.session |
Cleared on browser restart, extension reload, disable, or update. | Temporary state that must be available to extension contexts, including the service worker, during the current session. Chrome recommends it for sensitive user data. |
storage.sync |
Intended for settings that follow the user across signed-in Chrome browsers; subject to API quotas. | Small preferences that should sync, rather than large exports or general file storage. |
Quota limits are implementation details that can change; check the current Chrome Storage API reference rather than treating a remembered number as permanent.
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chrome.downloads.download() starts a URL download and, on supported Chrome versions, resolves with a download ID. When both filename and saveAs are specified, the Save As dialog is pre-populated with that filename. A basic flow is:
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- Declare the
downloadspermission in the manifest. - Call
chrome.downloads.download()with a URL and, if known, a relativefilename. Choose an explicit collision policy where appropriate. - Keep the returned download ID if later events need to identify this download, and persist it if it must survive service-worker shutdown.
- Observe
chrome.downloads.onChangedto track changes such as state and filename. Download states includein_progress,interrupted, andcomplete.
An interrupted download is not always an extension bug: Chrome represents file-system, network, server, user, and security interruptions with distinct documented error families. Inspect the changed state and error details before deciding what recovery is appropriate. The Downloads API reference also notes that HTTP and HTTPS requests include cookies set for the hostname. Do not treat the API as a generic sink for arbitrary in-memory blobs without validating the specific URL and data flow.
Choose storage by lifetime, sync needs, and sensitivity
chrome.storage is asynchronous and stores JSON-serializable values. Use storage.local when state should remain on the device across sessions, storage.session when it should end with the current session, and storage.sync for settings intended to follow a signed-in user across synced Chrome browsers. Chrome specifically recommends storage.session for sensitive user data. Use the current Storage API reference to check quotas and behaviors for your target Chrome version.
Check compatibility against the specific API
Chrome’s API index describes Manifest V3 as generally supported in Chrome 88 or later, but that does not establish that every API feature works in every version from Chrome 88 onward. Verify the minimum Chrome version for the specific Downloads or Storage feature you use in its API reference, and state that version when documenting compatibility.
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