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Anton Golub
Anton Golub

Posted on • Edited on

config: apply extends

#javascript #config #showdev

As we know, the forms and approaches to describing configuration files are extremely varied. I discussed this in the previous note. But there are several techniques and practices that may one day become standards.

One of this – the extends directive. Many tools provide this feature, but it works a little differently in each place. For example, tsconfig applies deep merge to compilerOptions, while prettier concatenates overrides array sections, commitlint joins plugin, etc. So the authors have to implement these nuances on site every time, and it's tiring and annoying a bit. I believe we will come up with a more efficient way to handle this routine.

Proposal 

import { populate, populateSync } from '@topoconfig/extends'

const tsconfig = await populate('tsconfig.json', {
  compilerOptions: 'merge'
})
const prettierCfg = populateSync('.prettierrc.json', {
  overrides: 'merge'
})
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20 minutes adventure

At first glance, the extends may seem like a simple combination of Object.assign and fs.readFile. Well, technically it is, but not quite so. Let's see how the real scenario unfolds. To simplify the details, we will use pseudocode.

The starting point:

input = {
  foo: 'bar',
  extends: './base.json'
}
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  • pick extends 
resource = input.extends
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  • load the referenced ./base.json 
extra = JSON.parse(fs.readFileSync(resource, 'utf8'))
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  • merge input and extra 
result = {
  ...input,
  ...extra,
  extends: undefined,
}
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How about ./base.js? Reasonable request.

  • provide js files support 
input = { extends: './base.js' }
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require hurries to the resque:

load = (ref) => isJs(ref) ? require(ref) : JSON.parse(fs.readFileSync(ref, 'utf8'))
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./base.mjs?

  • Ooops. Here the API switches to async mode to handle mjs/esm inputs. Of course, we can patch require with esm package, or load module via a process.spawnSynced nodejs. We can, but we won't 
load = isJs(ref)
  ? (await import(ref))?.default
  : fs.promises.readFile(ref, 'utf8')
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What if the value is passed without an extension, like base?

  • add support for shared npm configs
  • provide configurable resolve 
resolve = (ref, cwd) =>
  ref.startsWith('.') || ref.startsWith('/') || path.extname(ref)
    ? path.resolve(cwd, ref)
    : ref
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But what if extends is a nested object? It definitely is.

  • apply everything recursively: 
handle = input = ({
  ...input,
  ...handle(load(input.extends)),
  extends: undefined,
})
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In this case, circular references are something to pay attention to.

  • add index to avoid infinite loops 
memo = new Map()

memoizedLoad = ref => {
  if (memo.has(ref)) {
    return memo.get(ref)
  }
  memo.set(ref, load(ref))
  return load(ref)
}
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Relative resource path should use the closest parent dirname as a base to work properly.

  • resolve cwd for each extends reference 
base = path.resolve(process.cwd(), _opts.cwd ?? '.')
[cwd, _config] = typeof config === 'string'
  ? [path.resolve(base, path.dirname(config)), path.basename(config)]
  : [base, config]
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Multiple extends maybe?

  • let extends hold several references 
input = {
  foo: 'bar',
  extends: ['./extra.json', './mixin.js']
}
extras = [input.extends].flat().map(load)
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How to read toml, yaml, ini, env? Obviously, custom formats are required too.

  • provide parse option 
parse = (ref, contents) => {
  if (ref.endsWith('.yaml') || ref.endsWith('.yml')) 
      return parseYaml(contents)
  if (ref.endsWith('.json')) 
      return JSON.parse(contents)
  throw new Error(`Unsupported format: ${ref}`)
}
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How to resolve value conflicts: merge / override?

  • provide rules to handle prop-specific join strategies 
rules = {
  '*': 'merge',
  foo: 'override'
}
rule = getRule(propPath, rules)
result[key] = isObject(value) && rule === 'merge'
  ? extend({
    sources: [value],
    rules,
    prefix: p,
    index
  })
  : value
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What about arrays?

  • implement array merge as concat 
if (rule === 'merge') {
  arr.push(...sources.flat(1))
} else {
  arr.length = 0
  arr.push(...sources.slice(-1).flat(1))
}
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It would be nice to avoid mutations.

  • introduce clone 
safeLoad = compose(memoize, clone, load)
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Conclusion

Summarizing:

  • search extends statements
  • load & parse referenced resources
    • support async and sync flows
    • use cloning to avoid mutations
    • .js, .cjs, .mjs, .json
    • provide support for custom formats
    • resolve cwd for the nested resource by its closest parent resource
    • support custom resolvers
  • walk recursively
    • handle circular references
  • provide sync and async API
  • assemble the result
    • handle arrays / plain objects
    • apply prop-specific merge

Additionally:

  • build as cjs and esm
  • provide types
  • handle windows specific issues
  • add dozens of tests
  • handle different runtimes specifics (node, deno, bun)

There is an obvious benefit to implementing this algorithm on your own – it clarifies and makes controllable every aspect of config processing in place.
However, we do not always have enough time to do this exercise. Therefore, maybe someone will find this shortcut useful:

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