metro/docs/Bundling.md

id	title
bundling	Bundling

When bundling, each of the modules gets assigned a numeric id, meaning no dynamic requires are supported. Requires are changed by its numeric version, and modules are stored in different possible formats. Three different formats of bundling are supported:

Plain bundle

This is the standard bundling format. In this format, all files are wrapped with a function call, then added to the global file. This is useful for environments that expect a JS only bundle (e.g. a browser). Just requiring the entry point with the .bundle extension should trigger a build of it.

Indexed RAM bundle

This format composes the bundle as a binary file, which format has the following parts (all numbers are expressed in Little Endian):

• A magic number: a uint32 must be located at the beginning of the file, with the value 0xFB0BD1E5. This is used to verify the file.
• An offset table: the table is a sequence of uint32 pairs, with a header
  • For the header, two uint32s can be found: the length of the table, and the length of the startup code.
  • For the pairs, they represent the offset in the file and the length of code module, in bytes.
• Each of the modules, finished by a null byte (\0).

` 0                   1                   2                   3                   4                   5                   6
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Magic number                         |                          Header size                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Startup code size                       |                        Module 0 offset                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Module 0 length                        |                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                                               +
|                                                                                                                               |
+                                                              ...                                                              +
|                                                                                                                               |
+                                                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |                        Module n offset                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Module n length                        | Module 0 code | Module 0 code |      ...      |       \0      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Module 1 code | Module 1 code |      ...      |       \0      |                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                                               +
|                                                                                                                               |
+                                                              ...                                                              +
|                                                                                                                               |
+                                                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               | Module n code | Module n code |      ...      |       \0      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+`

This structure is optimal for an environment that is able to load all code in memory at once:

• By using the offset table, one can load any module in constant time, where the code for module x is located at file[(x + 3) * sizeof(uint32)]. Since there is a null character (\0) separating all modules, usually length does not even need to be used, and the module can be loaded directly as an ASCIIZ string.
• Startup code is always found at file[sizeof(uint32)].

This bundling is usually used by iOS.

File RAM bundle

Each module is stored as a file, with the name js-modules/${id}.js, plus an extra file called UNBUNDLE is created, which its only content is the magic number, 0xFB0BD1E5. Note that the UNBUNDLE file is created at the root. This bundling is usually used by Android, since package contents are zipped, and access to a zipped file is much faster. If the indexed format was used instead, all the bundled should be unzipped at once to get the code for the corresponding module.