Line number of SyntaxError in Node.js

Question

I have some code that uses vm module and runInNewContext function and executes dynamically generated JavaScript code. Basically a safer option of eval.

The code (variable code) can possibly contain syntax errors, so I would like to catch them and print some useful information.

try {
    vm.runInNewContext(code, sandbox, filename);
}
catch (e) {
    if (e instanceof SyntaxError) { // always false
        console.log(e.toString()); // "SyntaxError: Unexpected token ||" for example
        console.log(e.line); // how to get the line number?
    }
}

I'd like to print the number of the line with the syntax error, but I have two problems:

• I don't know how to recognize whether the exception is SyntaxError or something else. instaceof doesn't work (update - I can use e.name === "SyntaxError").
• Even if I was able to recognize it, how could I get the line number? Is it possible?

Thanks in advance.

Update: I can get some information from e.stack - however, the topmost call in the stack trace is runInNewContext (with its line number), but I still can't find the line number inside code, which caused the exception (SyntaxError).

Answer 1

use a try/catch

store the stack in a global variable

log the sandbox

var util = require('util');
var vm = require('vm');
var sandbox = {e:null};
var src = 'try{count += 1;}catch(err) {e=err.stack}';

vm.runInNewContext(src , sandbox, 'myfile.vm');
console.log(util.inspect(sandbox));

this will log:

{ e: 'ReferenceError: count is not defined\n at myfile.vm:1:13\n ...

now you can see that the error occurs at line 1, character 13 of myfile.vm this will still require you to place some try/cathc blocks at various places but at least you can get feedback now

EDIT:

Adding a method to detect Syntax errors from within your running code

var acorn = require('acorn');
var src = "try{\n"+
    "   {" +
    "       count += 1;\n"+
    "   }catch(err) {\n" +
    "       e=err.stack\n" +
    "}";

var result = acorn.parse(src);

doing this will result in a thrown Exception with "SyntaxError: Unexpected token (3:4)" eg. line 3, character 4 is the unexpected token, (the extra { that shouldnt be there)