在node.js中重新定义变量

Question

The execution of this script: tmp.js, that contains:

var parameters = {};
(1,eval)("var parameters = {a:1}");
(1,eval)(console.log(parameters));

node tmp.js

produces:

{}

If we comment out the first statement, and execute again the script, we obtain:

{ a: 1 }

The global scope contains exactly the same variables with the same value, so why console.log displays a different value?

Answer 1

Because all code you run in Node is running in a Node module ,¹ with its own scope, not at global scope. But the way you're calling eval (indirectly, (1,eval)(...) ) runs the code in the string at global scope. So you have two parameters variables: A local one in the module, and a global one. The local one wins (when it exists).

var parameters = {};                // <== Creates a local variable in the module
(1,eval)("var parameters = {a:1}"); // <== Creates a global variable
(1,eval)(console.log(parameters));  // <== Shows the local if there is one,
                                    //     the global if not

Your last line of code is a bit odd: It calls console.log , passing in parameters , and then passes the return value (which will be undefined ) into eval . Not much point in that call to eval with undefined.

If that last line were

(1,eval)("console.log(parameters)");

...it would run at global scope, not module scope, and always show the global.

Here's an example that does the same thing without Node:

 (function() { var parameters = {}; (1,eval)("var parameters = {a:1}"); console.log("local", parameters); (1,eval)('console.log("global", parameters);'); })(); 

---

¹ FWIW, according to the documentation , your code is wrapped in a function that looks like this:

(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});

...and then executed by Node.