The Tor Browser / file revision

 // Different representations of the same number or string are treated as the same Map key.

 var m = new Map;

 var test = function test(a, b) {

     m.set(a, 'secret');

     assertEq(m.get(b), 'secret');

     m.set(b, 'password');

     assertEq(m.get(a), 'password');

     assertEq(a, b);

 };

 // Float and integer representations of the same number are the same key.

 test(9, Math.sqrt(81));

 // Ordinary strings and ropes are the same key.

 var a = Array(1001).join('x');

 var b = Array(501).join('x') + Array(501).join('x');

 test(a, b);

 // Two structurally different ropes with the same characters are the same key.

 a = "";

 b = "";

 for (var i = 0; i < 10; i++) {

     a = Array(501).join('x') + a;

     b = b + Array(501).join('x');

 }

 test(a, b);