index.js

module.exports = {
  rules: {
    'selective-scope': (ctx, applicable, rule) => {
      if (applicable === 'never') {
        return [false, 'the "allowed-scopes" rule does not support "never"']
      }

      const allowedScopes = rule[ctx.type]

      // If the type does not appear in the rule config, allow any scope or no scope
      if (allowedScopes === undefined) {
        return [true]
      }

      if (Array.isArray(allowedScopes)) {
        // If the type maps to an empty array in the rule config, scope it not allowed
        if (allowedScopes.length === 0) {
          if (ctx.scope != null) {
            return [
              false,
              `commit messages with type "${ctx.type}" must not specify a scope`
            ]
          }

          return [true]
        }

        // Otherwise attempt to match against either null, a string literal, or a RegExp
        if (
          allowedScopes.findIndex(s => {
            if (
              typeof ctx.scope === 'string' &&
              Object.prototype.toString.call(s) === '[object RegExp]'
            ) {
              return ctx.scope.match(s)
            }

            // Equality comparison works for both strings and null
            return s === ctx.scope
          }) !== -1
        ) {
          return [true]
        } else if (allowedScopes.includes(null)) {
          return [
            false,
            `commit message with type "${
              ctx.type
            }" may specify a scope, but if specified, it must be one of the following: ${allowedScopes
              .filter(s => s !== null)
              .map(s => `"${s}"`)
              .join(', ')}`
          ]
        } else {
          return [
            false,
            `commit message with type "${
              ctx.type
            }" must specify one of the following scopes: ${allowedScopes
              .map(s => `"${s}"`)
              .join(', ')}`
          ]
        }
      }

      return [
        false,
        `invalid rule entry: { ${ctx.type}: ${JSON.stringify(allowedScopes)} }`
      ]
    }
  }
}