为什么不为undefined分配值会在现代浏览器中产生错误？

Question

For example imagine the following expression:

undefined = "whatever";

The value of undefined isn't change by this, but it doesn't produce any error or exception whatsoever either, neither in Firefox, Chrome, Edge nor IE11.

Actually the value of the expression is even the assigned value instead of undefined :

var x = (undefined = "whatever");

Now x holds the value "whatever" .

This seems weird on the one hand, but worse, a source for bugs on the other hand because nobody should even attempt to redefine undefined , and typos that result in doing so should be caught by the engine. Why would that fly?

Compare to the behavior when trying to redefine other keywords, for example:

for = 12345;

This yields "Uncaught SyntaxError: Unexpected token =" as it rightly should.

Answer 1

Because js has to be backwards compatible and at some time it was introduced that way. To tackle this problem strict mode was introduced. While using strict mode undefined = "whatever" will throw an error.

Answer 2

Reading from MDN :

While it is possible to use it as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a very bad idea that will make your code difficult to maintain and debug.

 (function () { var undefined = 'foo'; console.log(undefined, typeof undefined); })();