Read-eval-print loop (REPL) for Chrome App APIs.
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Read-eval-print loop (REPL) for Chrome App APIs.

Chrome packaged app that lets you play around with any of the app APIs.

Since packaged apps don't allow the direct use of eval(), implementing a REPL is a bit tricky. The app eval()s all code in a sandboxed frame. To enable API usage in a sandboxed frame (where APIs are normally disallowed), proxy/stub functions are created for all APIs such that they're executed in the main frame and the results forwarded back.


The REPL application is available in the Chrome Web Store. Use the "Add to Chrome" button from the store web page, and then launch it from the New Tab Page. Note that due to a Chrome bug you'll need to click on the console window to give it focus before you can start typing.

Built-in functions

To make life a bit easier, the REPL environment adds a few built-in functions that you can use.

  • log(), info(), warning(), and error() let you log values to the console at various logging levels (results of expressions are automatically logged)
  • serialize() is a helper method that logs a stringified serialization of the passed in values. This may be useful when inspecting complex objects returned by API methods.
  • _ is a magic placeholder value that can be passed in to all API methods and event listeners that expect a callback function. The _ will be replaced with a callback that logs its parameters via serialize()
  • ab2str and str2ab are helper methods for converting between ArrayBuffers and (ASCII-encoded) strings, which make interactions with the serial, socket, Bluetooth, and USB APIs easier.

You can get a complete list of built-in functions by running the help command. You can also the events and methods commands to list all API events that can be listener for and methods that can be invoked.


The implementation of the REPL relies on postMessage to communicate between the main and sandboxed frames. This is an asynchronous mechanism, and thus only asynchronous Chrome APIs can be proxied. Thankfully most APIs are asynchronous, but this does mean that some simple ones like chrome.runtime.getURL are not supported.

The proxying relies on parameters being copied from one context to another. This is done via the structured clone algorithm, which means that certain complex JavaScript types may not be supported. Functions parameters (used as callbacks) get special treatment and should work, but currently FileEntry values (as used by the chrome.fileSystem API) are not supported.

Sample usage

Here's a sample session that plays around with the socket API to do an HTTP request to via a TCP socket on port 80. This sesssion is also available as a screencast.

First, to save some typing, an alias for the socket API is created. The undefined that appears is the result of that expression (variable declarations don't return a value).

> var socket = chrome.socket;

Then a socket is created. The _ parameter is a special built-in magic value into the REPL that will generate a callback function that logs when it is called and dumps all of its parameters.

> socket.create('tcp', _);
chrome.socket.create callback invoked with:
  <object> {
      <number> 22

Now that we know the ID of the socket that was created (normally we would save it in the callback), we can use it to connect to on port 80 (the invocation of the callback with 0 indicates success).

> var socketId = 22;
> socket.connect(socketId, '', 80, _);
chrome.socket.connect callback invoked with:
  <number> 0

We then issue our HTTP request (str2ab is a built-in function that converts a string to an ArrayBuffer).

> socket.write(socketId, str2ab('GET / HTTP/1.0\r\n\r\n'), _);
chrome.socket.write callback invoked with:
  <object> {
      <number> 18

And then read the first 512 bytes of the response.

>, 512, _); callback invoked with:
  <object> {
      <ArrayBuffer, 512 bytes> HTTP/1.0 200 OK
Date: Mon, 22 Oct 2012 05:07:53 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Set-Cookie: PREF=ID=d8539cf2c8d07c94:FF=0:TM=1350882473:LM=1350882473:S=KfFPY0z1EOdqBstx; expires=Wed, 22-Oct-2014 05:07:53 GMT; path=/;
Set-Cookie: NID=65=DZuaw2vrI1YOsJl_K6Z9Rg_cYB2MrHyaVVLPC7ZpEnAhMFOGwq0GFCSCpGFenqTnD3tPmUXI7eS-UBpVI8COw_aGPHPxwFxEX_VC96P4DX5yb9t33AYnZnZgm0WJvhzE; expires=Tue, 23-Apr-2013 05:07:53 GMT; path=/;,
      <number> 512

Finally, we can close the socket.

> socket.disconnect(socketId);

Identity API setup

When running locally (as an unpacked app), the app's manifest.json should have the following snippet. It forces the app to have a stable ID (maekkhccpnonljjlbkejieamkbodeida) by giving it a key property (the value is chrome-app-repl base 64 encoded).

"key": "Y2hyb21lLWFwcC1yZXBs",
"oauth2": {
  "client_id": "",
  "scopes": [

The store version will have a different ID (omdkgkgnnakfiojpcjdobjgdlpimkcbp), so those keys should be replaced with:

"oauth2": {
  "client_id": "",
  "scopes": [