index.js
function lazyConsForce() {
  /* jshint validthis:true */
  var val = this.tailFn();
  /* eslint-disable no-use-before-define */
  this.tailValue = Array.isArray(val) ? fromArray(val) : val;
  /* eslint-enable no-use-before-define */

  delete this.tail;
  delete this.force;

  return this;
}
function lazyConsForce() {
  /* jshint validthis:true */
  var val = this.tailFn();
  /* eslint-disable no-use-before-define */
  this.tailValue = Array.isArray(val) ? fromArray(val) : val;
  /* eslint-enable no-use-before-define */

  delete this.tail;
  delete this.force;

  return this;
}
But when cons is created, it's overloaded with lazy ones Force tail to whnf.