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Implement CLI Config Precedence

Define a clear precedence of flags over environment over config file over defaults so behaviour is predictable.

const config = {
  ...defaults,
  ...readConfigFile(path),
  ...pickEnv(process.env),
  ...pickFlags(argv),
};

test('flag beats env', () => {
  expect(resolve({ FLAG: 'a' }, { FLAG: 'b' })).toBe('a');
});

What this recipe does

Merge CLI flags, environment variables, a config file, and defaults in a documented order.

Users are confused because an environment variable silently overrides their command-line flag.

Prerequisites

• A CLI with settings
• A config file format
• Environment variable conventions

Procedure

1. Merge sources in a fixed precedence order.

const config = {
  ...defaults,
  ...readConfigFile(path),
  ...pickEnv(process.env),
  ...pickFlags(argv),
};

Expected result: Flags win over environment, which wins over the file, which wins over defaults.

2. Document and test the order.

test('flag beats env', () => {
  expect(resolve({ FLAG: 'a' }, { FLAG: 'b' })).toBe('a');
});

Expected result: A test locks the precedence so refactors cannot silently change it.

How to confirm it worked

Run the commands above and confirm every expected result in order. If a step does not produce the stated output, use the troubleshooting notes before continuing. A passing run shows with all sources set, the flag value wins, and a test enforces the documented order.

Troubleshooting

Empty values override unintentionally — Treat empty strings as unset unless explicitly meaningful.

Config file not found — Fall back to defaults with a warning rather than failing hard.

Limitations

• Precedence must be documented for users.
• Complex merging of nested structures needs care.
• Does not cover secret storage.