iter-tools-es/impls/when/when.js

The iterable toolbox

const emptyIterator = {
  next() {
    return {
      value: undefined,
      done: true
    };
  }

};
const emptySpreadable = Object.freeze(Object.create({
  [Symbol.iterator]() {
    return emptyIterator;
  }

}));

function when(condition, value) {
  const valueType = typeof value;

  if (!(value == null || value[Symbol.iterator] || valueType === 'object' || valueType === 'function')) {
    throw new Error('Second argument to when must be an object, iterable, or function.');
  }

  return condition && value != null ? valueType === 'function' ? value() : value : emptySpreadable;
}

exports.when = when;