core-js/internals/check-correctness-of-iteration.js

'use strict';
var wellKnownSymbol = require('../internals/well-known-symbol');

var ITERATOR = wellKnownSymbol('iterator');
var SAFE_CLOSING = false;

try {
  var called = 0;
  var iteratorWithReturn = {
    next: function () {
      return { done: !!called++ };
    },
    'return': function () {
      SAFE_CLOSING = true;
    }
  };
  iteratorWithReturn[ITERATOR] = function () {
    return this;
  };
  // eslint-disable-next-line es/no-array-from, no-throw-literal -- required for testing
  Array.from(iteratorWithReturn, function () { throw 2; });
} catch (error) { /* empty */ }

module.exports = function (exec, SKIP_CLOSING) {
  try {
    if (!SKIP_CLOSING && !SAFE_CLOSING) return false;
  } catch (error) { return false; } // workaround of old WebKit + `eval` bug
  var ITERATION_SUPPORT = false;
  try {
    var object = {};
    object[ITERATOR] = function () {
      return {
        next: function () {
          return { done: ITERATION_SUPPORT = true };
        }
      };
    };
    exec(object);
  } catch (error) { /* empty */ }
  return ITERATION_SUPPORT;
};