js use "Chain of Responsibility Pattern" but after export, all external calls are empty

I am trying to implement "Chain of Responsibility Pattern" with JS, but the problem I encountered, after exporting "chainController", all external calls got NULL. From the code point of view, there is no asynchronous problem, which makes me very confused , please help me, very thankful

code:

/** Chain **/
const Chain = function(handler) {
  this.handler = handler;
  this.nextHandler = null;
}

Chain.prototype.setNextHandler = function(nextHandler) {
  this.nextHandler = nextHandler;
  return nextHandler;
}

Chain.prototype.pass = function(...args) {
  const result = this.handler(...args);
  if (result === 'next') {
    return this.nextHandler && this.nextHandler.pass(...args);
  }
  return result;
}

/** Handlers **/
const equalTen = function(number) {
  return (number === 10) ? 'equal-ten' : 'next';
}

const equalTwenty = function(number) {
  return (number === 20) ? 'equal-twenty' : 'next';
}

const equalThirty = function(number) {
  return (number === 30) ? 'equal-thirty' : 'next';
}

/** Controller **/
const chainController = function(number) {
  const checkEqualTen = new Chain(equalTen);
  const checkEqualTwenty = new Chain(equalTwenty);
  const checkEqualThirty = new Chain(equalThirty);

  checkEqualTen
    .setNextHandler(checkEqualTwenty)
    .setNextHandler(checkEqualThirty);

  return checkEqualTen.pass(number);
};

//is problem, when external call him, always get null
let test = chainController(15);
console.log(test);

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