This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

Add `default_converter` argument to `to_js` and `toJs`. This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

(quickstart)=

# Getting started

## Try it online

Try Pyodide in a [REPL](https://pyodide.org/en/latest/console.html) directly in your browser (no installation needed).

## Setup

To include Pyodide in your project you can use the following CDN URL:

```text
https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js
```

You can also download a release from [GitHub
releases](https://github.com/pyodide/pyodide/releases) or build Pyodide
yourself. See {ref}`downloading_deploying` for more details.

The `pyodide.js` file defines a single async function called
{any}`loadPyodide <globalThis.loadPyodide>` which sets up the Python environment
and returns {js:mod}`the Pyodide top level namespace <pyodide>`.

```pyodide
async function main() {
  let pyodide = await loadPyodide({ indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/" });
  // Pyodide is now ready to use...
  console.log(pyodide.runPython(`
    import sys
    sys.version
  `));
};
main();
```

## Running Python code

Python code is run using the {any}`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, translated to JavaScript objects (see
{ref}`type-translations`). For example the following code will return the
version string as a JavaScript string:

```pyodide
pyodide.runPython(`
  import sys
  sys.version
`);
```

After importing Pyodide, only packages from the standard library are available.
See {ref}`loading_packages` for information about loading additional packages.

## Complete example

Create and save a test `index.html` page with the following contents:

```html-pyodide
<!DOCTYPE html>
<html>
  <head>
      <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  </head>
  <body>
    Pyodide test page <br>
    Open your browser console to see Pyodide output
    <script type="text/javascript">
      async function main(){
        let pyodide = await loadPyodide({
          indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/"
        });
        console.log(pyodide.runPython(`
            import sys
            sys.version
        `));
        console.log(pyodide.runPython("print(1 + 2)"));
      }
      main();
    </script>
  </body>
</html>
```

## Alternative Example

```html
<!DOCTYPE html>
<html>
  <head>
    <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  </head>

  <body>
    <p>
      You can execute any Python code. Just enter something in the box below and
      click the button.
    </p>
    <input id="code" value="sum([1, 2, 3, 4, 5])" />
    <button onclick="evaluatePython()">Run</button>
    <br />
    <br />
    <div>Output:</div>
    <textarea id="output" style="width: 100%;" rows="6" disabled></textarea>

    <script>
      const output = document.getElementById("output");
      const code = document.getElementById("code");

      function addToOutput(s) {
        output.value += ">>>" + code.value + "\n" + s + "\n";
      }

      output.value = "Initializing...\n";
      // init Pyodide
      async function main() {
        let pyodide = await loadPyodide({
          indexURL: "https://cdn.jsdelivr.net/pyodide/dev/full/",
        });
        output.value += "Ready!\n";
        return pyodide;
      }
      let pyodideReadyPromise = main();

      async function evaluatePython() {
        let pyodide = await pyodideReadyPromise;
        try {
          let output = pyodide.runPython(code.value);
          addToOutput(output);
        } catch (err) {
          addToOutput(err);
        }
      }
    </script>
  </body>
</html>
```

## Accessing Python scope from JavaScript

You can also access from JavaScript all functions and variables defined in
Python by using the {any}`pyodide.globals` object.

For example, if you run the code `x = numpy.ones([3,3])` in Python, you can
access the variable `x` from JavaScript in your browser's developer console
as `pyodide.globals.get("x")`. The same goes
for functions and imports. See {ref}`type-translations` for more details.

You can try it yourself in the browser console:

```pyodide
pyodide.runPython(`
  import numpy
  x=numpy.ones((3, 4))
`);
pyodide.globals.get('x').toJs();
// >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]

// create the same 3x4 ndarray from js
x = pyodide.globals.get('numpy').ones(new Int32Array([3, 4])).toJs();
// x >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]
```

Since you have full access to Python global scope, you can also re-assign new
values or even JavaScript functions to variables, and create new ones from
JavaScript:

```pyodide
// re-assign a new value to an existing variable
pyodide.globals.set("x", 'x will be now string');

// create a new js function that will be available from Python
// this will show a browser alert if the function is called from Python
pyodide.globals.set("alert", alert);

// this new function will also be available in Python and will return the squared value.
pyodide.globals.set("square", x => x*x);

// You can test your new Python function in the console by running
pyodide.runPython("square(3)");
```

Feel free to play around with the code using the browser console and the above example.

## Accessing JavaScript scope from Python

The JavaScript scope can be accessed from Python using the `js` module (see
{ref}`type-translations_using-js-obj-from-py`). This module represents the
global object `window` that allows us to directly manipulate the DOM and access
global variables and functions from Python.

```python
import js

div = js.document.createElement("div")
div.innerHTML = "<h1>This element was created from Python</h1>"
js.document.body.prepend(div)
```

(quickstart)=

# Getting started

## Try it online

Try Pyodide in a [REPL](https://pyodide.org/en/latest/console.html) directly in your browser (no installation needed).

## Setup

To include Pyodide in your project you can use the following CDN URL:

```text
https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js
```

You can also download a release from [GitHub
releases](https://github.com/pyodide/pyodide/releases) or build Pyodide
yourself. See {ref}`downloading_deploying` for more details.

The `pyodide.js` file defines a single async function called
{any}`loadPyodide <globalThis.loadPyodide>` which sets up the Python environment
and returns {js:mod}`the Pyodide top level namespace <pyodide>`.

```pyodide
async function main() {
  let pyodide = await loadPyodide({ indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/" });
  // Pyodide is now ready to use...
  console.log(pyodide.runPython(`
    import sys
    sys.version
  `));
};
main();
```

## Running Python code

Python code is run using the {any}`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, translated to JavaScript objects (see
{ref}`type-translations`). For example the following code will return the
version string as a JavaScript string:

```pyodide
pyodide.runPython(`
  import sys
  sys.version
`);
```

After importing Pyodide, only packages from the standard library are available.
See {ref}`loading_packages` for information about loading additional packages.

## Complete example

Create and save a test `index.html` page with the following contents:

```html-pyodide
<!DOCTYPE html>
<html>
  <head>
      <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  </head>
  <body>
    Pyodide test page <br>
    Open your browser console to see Pyodide output
    <script type="text/javascript">
      async function main(){
        let pyodide = await loadPyodide({
          indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/"
        });
        console.log(pyodide.runPython(`
            import sys
            sys.version
        `));
        console.log(pyodide.runPython("print(1 + 2)"));
      }
      main();
    </script>
  </body>
</html>
```

## Alternative Example

```html
<!DOCTYPE html>
<html>
  <head>
    <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  </head>

  <body>
    <p>
      You can execute any Python code. Just enter something in the box below and
      click the button.
    </p>
    <input id="code" value="sum([1, 2, 3, 4, 5])" />
    <button onclick="evaluatePython()">Run</button>
    <br />
    <br />
    <div>Output:</div>
    <textarea id="output" style="width: 100%;" rows="6" disabled></textarea>

    <script>
      const output = document.getElementById("output");
      const code = document.getElementById("code");

      function addToOutput(s) {
        output.value += ">>>" + code.value + "\n" + s + "\n";
      }

      output.value = "Initializing...\n";
      // init Pyodide
      async function main() {
        let pyodide = await loadPyodide({
          indexURL: "https://cdn.jsdelivr.net/pyodide/dev/full/",
        });
        output.value += "Ready!\n";
        return pyodide;
      }
      let pyodideReadyPromise = main();

      async function evaluatePython() {
        let pyodide = await pyodideReadyPromise;
        try {
          let output = pyodide.runPython(code.value);
          addToOutput(output);
        } catch (err) {
          addToOutput(err);
        }
      }
    </script>
  </body>
</html>
```

## Accessing Python scope from JavaScript

You can also access from JavaScript all functions and variables defined in
Python by using the {any}`pyodide.globals` object.

For example, if you run the code `x = numpy.ones([3,3])` in Python, you can
access the variable `x` from JavaScript in your browser's developer console
as `pyodide.globals.get("x")`. The same goes
for functions and imports. See {ref}`type-translations` for more details.

You can try it yourself in the browser console:

```pyodide
pyodide.runPython(`
  import numpy
  x=numpy.ones((3, 4))
`);
pyodide.globals.get('x').toJs();
// >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]

// create the same 3x4 ndarray from js
x = pyodide.globals.get('numpy').ones(new Int32Array([3, 4])).toJs();
// x >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]
```

Since you have full access to Python global scope, you can also re-assign new
values or even JavaScript functions to variables, and create new ones from
JavaScript:

```pyodide
// re-assign a new value to an existing variable
pyodide.globals.set("x", 'x will be now string');

// create a new js function that will be available from Python
// this will show a browser alert if the function is called from Python
pyodide.globals.set("alert", alert);

// this new function will also be available in Python and will return the squared value.
pyodide.globals.set("square", x => x*x);

// You can test your new Python function in the console by running
pyodide.runPython("square(3)");
```

Feel free to play around with the code using the browser console and the above example.

## Accessing JavaScript scope from Python

The JavaScript scope can be accessed from Python using the `js` module (see
{ref}`type-translations_using-js-obj-from-py`). This module represents the
global object `window` that allows us to directly manipulate the DOM and access
global variables and functions from Python.

```python
import js

div = js.document.createElement("div")
div.innerHTML = "<h1>This element was created from Python</h1>"
js.document.body.prepend(div)
```
