# Using Pyodide from Javascript

This document describes using Pyodide directly from Javascript. For information
about using Pyodide from Iodide, see [Using Pyodide from
Iodide](using_pyodide_from_iodide.md).

## Startup

Include `pyodide.js` in your project.

This has a single `Promise` object which bootstraps the Python environment:
`languagePluginLoader`. Since this must happen asynchronously, it is a
`Promise`, which you must call `then` on to complete initialization. When the
promise resolves, pyodide will have installed a namespace in global scope:
`pyodide`.

```javascript
languagePluginLoader.then(() => {
  // pyodide is now ready to use...
  console.log(pyodide.runPython('import sys\nsys.version'));
});
```

## Running Python code

Python code is run using the `pyodide.runPython` function. It takes as input a
string of Python code. If the code ends in an expression, it returns the result
of the expression, converted to Javascript objects (See [type
conversions](type_conversions.md)).

```javascript
pyodide.runPython('import sys\nsys.version'));
```

## Loading packages

Only the Python standard library and `six` are available after importing
Pyodide. To use other libraries, you'll need to load their package using
`pyodide.loadPackage`. This downloads the file data over the network (as a
`.data` and `.js` index file) and installs the files in the virtual filesystem.

Packages can be loaded by name, for those included in the official pyodide
repository (e.g. `pyodide.loadPackage('numpy')`). It is also possible to load
packages from custom URLs (e.g.
`pyodide.loadPackage('https://foo/bar/numpy.js')`), in which case the URL must
end with `<package-name>.js`.

When you request a package from the official repository, all of that package's
dependencies are also loaded. Dependency resolution is not yet implemented
when loading packages from custom URLs.

Multiple packages can also be loaded in a single call,
```js
pyodide.loadPackage(['cycler', 'pytz'])
```

`pyodide.loadPackage` returns a `Promise`.

```javascript
pyodide.loadPackage('matplotlib').then(() => {
  // matplotlib is not available
});
```

## Complete example

TODO