Evaluate JavaScript and TypeScript snippets in markdown files.
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.
We start with discussing making use of the tool from the command line, but later sections cover authoring snippets and validating them post annotation.
The sections below discuss configuring the tool and authoring of examples.
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.
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
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
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
npx evaldown --config <path_to_config>
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"
};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.
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"
};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"
};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.
The most convenient way to capture the console is usually to add a flag to the particular snippet (this is discussed below). If all the examples require the console, it can be enabled globally via an option to the CLI:
npx evaldown --capture console ...
or by adding an outputCapture key to the configuration object:
module.exports = {
outputCapture: "console",
sourcePath: "./input",
targetPath: "./output"
};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
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 subsequent output block:
```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' }
When authoring examples you may find that you want to customise how individual snippets are treated - this is done by annotations above the code clocks inline in the markdown.
Comments with the evaldown marker will be located and the values
afterwards, which we call flags, will be used as processing hints.
This flag permits the use of the await keyword:
```js
const value = await Promise.resolve('foo');
return `${value}bar`;
```
```output
'foobar'
```
This flag will instrument the console such that any logged lines will appear in the output snippet:
```js
console.warn("whoa there!");
```
```output
whoa there!
```
Once you have authored your documentation and annotated the markdown such that it can be executed, it is important that as your project develops these examples stay correct with respect to future changes.
We can make use of the ability to execute the snippets to check that the
evaluated output matches what was previously recorded in the markdown. To
do this we can use the --validate option:
npx evaldown --validate ./README.md
In the case of an output mismatch an informative diff is produced:
SnippetValidationError:
expected \'foo\' to equal \'foobar\'
-foo
+foobar
This option allows such checks to easily occur as part of CI pipelines.