Lab

Factory

Three callers, one factory

The lesson asked what the CLI gets if the factory merged with a spread and the CLI passed outDir: undefined. Make exactly the change below, the lesson’s snippet, then run it: the cli line answers the prediction. Restore code puts it back.

// Lines 7–13 (the return statement) become a spread:
const defaults = {
  mode,
  target: 'es2022',
  format: preset === 'library' ? 'esm' : 'iife',
  minify: mode === 'production',
  outDir: `dist/${preset}`,
};
return { ...defaults, ...overrides };

// And the CLI, run without its --out flag, passes outDir: undefined (line 17):
const cli = resolveBuildConfig('app', { outDir: undefined });

What to look forA caller’s answer, and the default it meets

A spread copies every own property, undefined included, so an outDir key that is present but undefined overwrites the default: the CLI gets outDir undefined. The factory as written asks per field with ??, which falls back for undefined and null and keeps everything else, false included: the release task still gets minify=false.

type Overrides = { mode?: string; minify?: boolean; outDir?: string };
type BuildConfig = { mode: string; target: string; format: string; minify: boolean; outDir: string };

// The factory: the preset's rules, the defaults and the caller's overrides, in one place.
function resolveBuildConfig(preset: string, overrides: Overrides = {}): BuildConfig {
  const mode = overrides.mode ?? 'development';
  return {
    mode,
    target: 'es2022',
    format: preset === 'library' ? 'esm' : 'iife',
    minify: overrides.minify ?? mode === 'production',
    outDir: overrides.outDir ?? `dist/${preset}`,
  };
}

// Three callers, each saying only what it knows.
const cli = resolveBuildConfig('app', { outDir: 'build' });
const watch = resolveBuildConfig('library');
const release = resolveBuildConfig('library', { mode: 'production', minify: false });

const show = (name: string, config: BuildConfig) =>
  console.log(`${name}: ${config.mode} ${config.target} ${config.format} minify=${config.minify} ${config.outDir}`);
show('cli', cli);
show('watch', watch);
show('release', release);

CallerCLI No result observed yet

CallerWatch No result observed yet

CallerRelease No result observed yet

Configuration factoryresolveBuildConfig

Observed request · captured on entrySelected request → —

Waiting for a request

Rules owned here · lesson annotation

Preset → target and format

Mode → minify default

Output root → directory default

An explicit choice can replace a default.

Factory calls observed0

Observed configurationLatest observed values

ModeCaller or mode default —

TargetShared / preset rule —

FormatPreset rule —

MinifyCaller or mode default —

Output directoryCaller or output-root default —

Outlined rows: a field was supplied in the request. Values can be partial during creation or rejection; they show what was recorded.