node_modules/core-js/modules/_to-primitive.js

// 7.1.1 ToPrimitive(input [, PreferredType])
var isObject = require('./_is-object');
// instead of the ES6 spec version, we didn't implement @@toPrimitive case
// and the second argument - flag - preferred type is a string
module.exports = function (it, S) {
  if (!isObject(it)) return it;
  var fn, val;
  if (S && typeof (fn = it.toString) == 'function' && !isObject(val = fn.call(it))) return val;
  if (typeof (fn = it.valueOf) == 'function' && !isObject(val = fn.call(it))) return val;
  if (!S && typeof (fn = it.toString) == 'function' && !isObject(val = fn.call(it))) return val;
  throw TypeError("Can't convert object to primitive value");
};