vm: variable declaration persists across two runInContext()

[abenhamdine]

• Version: 5.7.0
• Platform: windows 7.1 Pro 64 bits
• Subsystem: vm

Code

const vm = require('vm');
const assert = require('assert');
const context = {
    results: {}
};

// 1st execution
const sandBox1 = vm.createContext(context);

let sFormula1 = "";
sFormula1 += "'use strict';\n";
sFormula1 += "let x = 1;\n";
sFormula1 += "let y = 2;\n";
sFormula1 += "let z = 0 ;\n";
sFormula1 += "z = x + y;\n";
sFormula1 += "results.z = z;\n";

vm.runInContext(sFormula1, sandBox1);

// *** 2nd execution ***
// get the value of z from the first sandbox
context.results.z = sandBox1.results.z;
// sandboxing the new context
const sandBox2 = vm.createContext(context);

let sFormula2 = "";
sFormula2 += "'use strict';\n";
sFormula2 += "let z = 10\n";
sFormula2 += "results.z = z;\n";

vm.runInContext(sFormula2, sandBox2);

Expected result

This code should't throw.

Actual result

This code throw the following error:

evalmachine.<anonymous>:1
'use strict';
^

TypeError: Identifier 'z' has already been declared
    at evalmachine.<anonymous>:1:1
    at Object.exports.runInContext (vm.js:44:17)
    at Object.<anonymous> (C:\node-projects\cvl-node-extdirect\bugVM.js:33:4)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:141:18)
    at node.js:933:3

let declaration is block-scope, and note that it's not even the same sandbox in the two runInContext(), so how is it possible to get this error ?

Am I wrong somewhere ?

[cjihrig]

sandBox1, sandBox2, and context are all the same object. vm.createContext() returns the object that is passed to it.

[abenhamdine]

@cjihrig Ah thanks I did'nt get that !
But I'm still a bit confused even after reading the doc

OK, sandBox1 and sandBox2 are the same object.

But in the first call to runInContext(), z has been declared local :
let z = 0;
So the context should'nt be modified, unlike if z has been declared global z = 0 or var z = 0 and despite this, z already exist when runInContext() is called for the second time.

So I'm missing a point here :

• does it mean that, if I pass the same sandbox object to 2 differents runInContext(), the 2 "run" will share the same context and the same variables ?
• if yes, does it mean then createContext() register somewhere a context, associated with a given sandbox ? The doc is not very explicit on this.

Thx in advance for any clarification !

If you think it's useful, I wil send a PR to make the doc more explicit on this.

[cjihrig]

This issue might be relevant to what you're seeing - #983.

does it mean that, if I pass the same sandbox object to 2 differents runInContext(), the 2 "run" will share the same context and the same variables ?

Yes.

if yes, does it mean then createContext() register somewhere a context, associated with a given sandbox ? The doc is not very explicit on this.

Yes. See lib/vm.js and src/node_contextify.cc.

Documentation PRs are almost always a good thing, so feel free to make one.

[abenhamdine]

@cjihrig thx again for your explanation ! It's more clear now, I will send a doc PR soon.

[bnoordhuis]

I'll close, looks like it's resolved? Let me know if I should reopen.

[abenhamdine]

Yes, it's resolved. A doc improvement would be a good thing IMHO, I will send a PR when I have some time, but it's definitely not a bug.