Function chain

packages/angular_devkit/schematics/src/rules/base.ts:35–50

(rules: Iterable<Rule> | AsyncIterable<Rule>)

 * Chain multiple rules into a single rule.
 */
export function chain(rules: Iterable<Rule> | AsyncIterable<Rule>): Rule {
  return async (initialTree, context) => {
    let intermediateTree: Observable<Tree> | undefined;
    if (Symbol.asyncIterator in rules) {
      for await (const rule of rules) {
        intermediateTree = callRule(rule, intermediateTree ?? initialTree, context);
      }
    } else {
      for (const rule of rules) {
        intermediateTree = callRule(rule, intermediateTree ?? initialTree, context);
      }
    }

    return () => intermediateTree;
  };
}

/**
 * Apply multiple rules to a source, and returns the source transformed.