'eval var' in class method not work

[andersonlunog]

• Version: v10.15.3
• Platform: Linux nogueira-notebook 4.15.0-50-generic npm install is unbelievably slow, fails in certain environments #54-Ubuntu SMP Mon May 6 18:46:08 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem:

Code:

function compareOut(obj, pattern) {
  for (const k in obj) {
    eval(`var ${k} = obj.${k};`);
  }
  return eval(`${pattern};`);
}

class Comparator {
  compareIn(obj, pattern) {
    for (const k in obj) {
      eval(`var ${k} = obj.${k};`);
    }
    return eval(`${pattern};`);
  }
  compare() {
    var pattern = 'name == "Nogueira"';
    var obj = {
      name: "Nogueira"
    }
    try {
      console.log("Result Test 1: " + compareOut(obj, pattern));
      console.log("Result Test 2: " + this.compareIn(obj, pattern));
    } catch (error) {
      console.error(error);
    }
  }
}

var c = new Comparator();
c.compare();

Expected result

Result Test 1: true
Result Test 2: true

But received

Result Test 1: true
ReferenceError: name is not defined

[mscdex]

Class methods are implicitly executed in strict mode and in strict mode eval() only creates variables that are available within the code passed to eval(). That is why you see the ReferenceError.

[retorquere]

So this in the main script scope should work?

eval('const x = 1');
console.log(x);

because that still gives me a ReferenceError.

[Trott]

So this in the main script scope should work?

eval('const x = 1');
console.log(x);

because that still gives me a ReferenceError.

That should give you a ReferenceError. x in the console.log() call doesn't know anything about x in the eval() call. const scoping is much more narrow than var. Change it to var and it will work. This has nothing to do with Node.js specifically. The browser console will show the same results.

[retorquere]

Ah, I thought eval ran the code in the current scope, but it doens't fully then?

[Trott]

Ah, I thought eval ran the code in the current scope, but it doens't fully then?

IIUC, it runs in the current local scope (unless, for example, invoked by a reference, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Description) but it creates its own block scope inside of that. So const and let do not leak, but var does.