evaldown
Evalute JavaScript snippets in markdown files and output static pages.
Last updated a month ago by alexjeffburke .
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Evaldown

Evaluate JavaScript and TypeScript snippets in markdown files.

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This tool provides both CLI and programmatic interfaces for locating JS/TS code blocks in one or more markdown files, extracting and evaluating these blocks and provides a range formats in which to serialise their pretty-printed output.

Use

We start by introducing an invocation for processing a single markdown file:

npx evaldown ./docs/README.md > README.md

The file will be processed and the output written to stdout. In order to store the output within the source file, thereby automatically capturing it, we can use the --inplace option:

npx evaldown --inplace ./docs/README.md

All the examples in this section are executable in a checkout of the evaldown repository.

Process directories of files

Processing all the files in a directory looks almost identical:

npx evaldown --target-path testdata/output testdata/example

As does applying an update to the source files within a directory:

npx evaldown --inplace ./testdata/example

Working with TypeScript

Support is inbuilt for processing TypeScript blocks into files. An explicit path to the tsconfig.json file is required from which point the project specific compiler is detected and used to transpile snippets:

npx evaldown --tsconfig-path ./testdata/typescript/tsconfig.json ./testdata/typescript/example.md

Beyond command line options

The tool supports many additional options to alter its behaviour.

Typically, the tool would be installed via a dependency via npm and any options will be read directly from a configuration file:

npm install --save-dev evaldown
./node_modules/.bin/evaldown --config <path_to_config>

The sections below discuss configuring the tool and authoring of examples.

Configuration

The tool ships with inbuilt support for processing directories of markdown files. To do this, a small config file is needed to indicate where the source path to read files from a target path to write generated output to.

A basic evaldown.conf.js file is as follows:

module.exports = {
  sourcePath: "./input",
  targetPath: "./output"
};

Output format and extension

Currently the rendering process will produce HTML files as standard with their usual .html file extension. The tool can however be requested to output markdown files to the output directory - with the output blocks populated - allowing its use to pre-process markdown files before they are passed to another template engine.

"inlined"

This option will write markdown files with the code and output blocks replaced with static HTML that inlines all the colouring information.

module.exports = {
  outputFormat: "inlined",
  sourcePath: "./input",
  targetPath: "./output"
};

"markdown"

This option will write markdown files with the code and output blocks replaced with text (for use when external highlighting is desired).

module.exports = {
  outputFormat: "markdown",
  sourcePath: "./input",
  targetPath: "./output"
};

Capturing evaluation results from the console

By default, JavaScript code blocks found in markdown files - which we refer to as snippets - are allowed to use return statements. The returned values will be rendered as an output block - an example of this is shown in the authoring section below.

In some cases, rather than capture the result you may wish to capture the logging output of a command, perhaps for code that emits messages when it finished or just an example that uses the console.

Capturing from the console can be configured by adding an outputCapture key with a value of "console" to the configuration object:

module.exports = {
  outputCapture: "console",
  sourcePath: "./input",
  targetPath: "./output"
};

Keeping the source up-to-date

As you change your examples, updating means you can always keep the output up-to-date. This mode is considered a key use-case and can enabled by default via the configuration file:

It can also be activaited on the command line on demand:

./node_modules/.bin/evaldown --config <path_to_config> --update

Authoring

Inside the input folder, you can make add markdown files that contain "javascript" code blocks. In order to have any output shown these need to be followed by "output" snippets.

By default, value returned from the code block is what will be captured and displayed in the

```javascript
function doSomething() {
  return { foo: "bar" };
}

// objects are inspected too
return doSomething();
```

```output
{ foo: 'bar' }
```

When they are rendered, the output will look something like:

function doSomething() {
  return { foo: "bar" };
}

// objects are inspected too
return doSomething();
{ foo: 'bar' }

Customising snippets

When authoring examples you may find that you want to customise how individual snippets are treated - be this to allow using promises or to capture the console.

HTML comments inserted above the code blocks allow doing just this. First, we look at an example that makes use of some async code:

```js
return Promise.resolve('foo');
```

```output
'foo'
```

Comments with the evaldown marker will be located and the values afterwards, which we call flags, will be used as processing hints.

Outputting uses of the console would look something like:

```js
console.warn("whoa there!");
```

```output
'whoa there!'
```

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