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node_modules/core-js/modules/esnext.map.key-of.js

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+'use strict';
+var $ = require('../internals/export');
+var aMap = require('../internals/a-map');
+var iterate = require('../internals/map-iterate');
+
+// `Map.prototype.keyOf` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'Map', proto: true, real: true, forced: true }, {
+  keyOf: function keyOf(searchElement) {
+    var result = iterate(aMap(this), function (value, key) {
+      if (value === searchElement) return { key: key };
+    }, true);
+    return result && result.key;
+  }
+});