Bug 275145 – Incorrect source location for TDZ error in modules when thrown before other statements

• Status: NEW
• Component: JavaScriptCore

[Jarred Sumner]

For the following input code:
```
// A statement which doesn't declare any variables
Intl;








const a = a;
```

When running `jsc` (correct, not a module):

```
jsc error.js

Exception: ReferenceError: Cannot access uninitialized variable.
global [e-mail]:11:12
```

When running `jsc -m` (incorrect, is a module):

```
jsc -m error.js

Exception: ReferenceError: Cannot access uninitialized variable.
module [e-mail]:2:5
```

It's pointing to the first statement in the function body instead of the location where it happened. 

Relevant issue: https://github.com/oven-sh/bun/issues/6824

[Jarred Sumner]

One way to fix it: call `emitExpressionInfo` before the `emitTDZCheckIfNecessary` call in `ResolveNode::emitBytecode`:

https://github.com/WebKit/WebKit/blob/d7759dc9cbb90b3fda92a54521c132c18cc5a0df/Source/JavaScriptCore/bytecompiler/NodesCodegen.cpp#L263
```
        JSTextPosition divot = m_start + m_ident.length();
        generator.emitExpressionInfo(divot, m_start, divot);
```

I'm guessing this is hot code though. There's probably a way to do this only when  no expression info has been emitted before

[Jarred Sumner]

Similar behavior occurs for `emitReadOnlyExceptionIfNeeded`

The following input:

```
const a = 1;
print(a);
a = 2;
```

Emits the following error:
```
❯ jsc -m /Users/jarred/Code/bun/src/bun.js/WebKit/a.js
1
Exception: TypeError: Attempted to assign to readonly property.
module code@/Users/jarred/Code/bun/src/bun.js/WebKit/a.js:2:6
```

The reported line number should be 3, which it is when not a module.

[Radar WebKit Bug Importer]

<rdar://problem/129316361>

[Ahmad Saleem]

@Jarred - is it regression by any chance (if you know)?

[Jarred Sumner]

@Ahmad I don't think this ever worked (for modules). We figured it was a bug in Bun because we had various bugs with sourcemaps, but we fixed the sourcemaps bug and this reproduces in the jsc shell.