Javascript stack trace line numbers for code within new Function are off by 2:
const fn = new Function(` // line 1
try { // line 2
blah // line 3
} catch(e) { // line 4
return e.stack // line 5
} // line 6
`)
console.log(fn())Running the above snippet produces:
anonymous@https://stacksnippets.net/js line 12 > Function:5:5
where it should say Function:3:5. Same thing when run in node:
$ cat stack-trace-line-nums.js
const fn = new Function(` // line 1
try { // line 2
blah // line 3
} catch(e) { // line 4
return e.stack // line 5
} // line 6
`)
console.log(fn())
$ node stack-trace-line-nums.js
ReferenceError: blah is not defined
at eval (eval at <anonymous> (/tmp/stack-trace-line-nums.js:1:12), <anonymous>:5:5)
...
The most obvious way to demonstrate is to put all the code on the same line:
const fn = new Function('try { blah } catch(e) { return e.stack }')
console.log(fn())which produces:
anonymous@https://stacksnippets.net/js line 12 > Function:3:7
instead of Function:1:7.
Any idea what's the cause or if there's a workaround?
Log the function itself and you'll see what's going on:
const fn = new Function(` // line 1
try { // line 2
blah // line 3
} catch(e) { // line 4
return e.stack // line 5
} // line 6
`)
console.log(fn);
console.log(fn())It actually becomes:
function anonymous(
) {
// line 1
try { // line 2
The first line is the function definition, the second line is the end of the parameter list and the start of the block, and only on line 3 does the actual body of the function passed into the constructor get inserted.
So, just add 2 to the line number every time.