test-readme-example.js

// Reads JS from stdin (the code of the readme example being tested), resolves
// module name requires to the main file instead, adds a require if not
// present, and checks that when it is run, its stdout matches the total of all
// comments of something like the form `// -> desired output`.

const fs = require('fs')
const { spawnSync } = require('child_process')
const { name: moduleName, main: mainFileName } = require('./package.json')

const moduleRequireCallPattern = `require\\(('${moduleName}'|"${moduleName}")\\)`
const moduleNameWhenImplicitlyRequired = 'ArrayKeyedMap'

// Load code from stdin
let code = fs.readFileSync(0, 'utf-8').toString()

const outputSpecs = code.matchAll(/\/\/ (?:=>|->|→|⇒) (.*)/g)
const requiredStdout = [...outputSpecs].map(x => x[1] + '\n').join('')
console.log({ requiredStdout })

// If the code didn't require this module, add a require in the beginning
if (!code.match(moduleRequireCallPattern)) {
  code = `const ${moduleNameWhenImplicitlyRequired} = require('./${mainFileName}')\n${code}`
} else {
  // Replace all occurrences of requiring this module by name, by requiring
  // this module by its main file
  code = code.replace(
    new RegExp(moduleRequireCallPattern, 'g'),
    `require('./${mainFileName}')`)
}

const { stdout, stderr, status } = spawnSync(
  'node', { input: code, encoding: 'utf-8' })

console.log({ stdout, stderr, status })

if (stdout !== requiredStdout) {
  console.log('Stdout did not match comments specifying output')
  process.exit(1)
} else {
  if (status) {
    process.exit(status)
  }
}