babel-plugin-transform-catch-scoping

A Babel plugin that fixes the JScript (IE6/7/8) catch scope bug by automatically renaming conflicting catch parameters and registering them in the enclosing function scope.

When to Use This Plugin?

Recommended Workflow

| Scenario | Solution | Advantages | |---------------------------|-----------------------------------|-------------------------------------| | Production builds | Terser with ie8: true | Smaller bundle + IE8 fixes | | Development debugging | This plugin | Readable code with IE8 compatibility|

⚠️ Note: Terser's ie8 option already handles catch scope renaming in production builds. Use this plugin only if you need uncompressed IE8-compatible code during development.

The Problem

In IE6/7/8 (JScript), the catch parameter does not have block scope — it leaks into the enclosing function scope, shadowing any variable with the same name:

function () {
  var e = '123';
  try {
    throw new Error();
  } catch (e) {
    console.error(e);
  }
  console.log(e); // IE8: Error object! Modern browsers: '123'
}

What This Plugin Does

This plugin detects when a catch parameter name conflicts with a binding or reference in the enclosing function scope, and renames the parameter to avoid the conflict:

// Before
function () {
  var e = '123';
  try { } catch (e) { console.error(e); }
  console.log(e);
}

// After
function () {
  var e = '123';
  try { } catch (_e) { console.error(_e); }
  console.log(e);
}

It also registers the catch parameter name in the function scope so that other plugins (like @babel/plugin-transform-block-scoping) will avoid generating variables with the same name.

Installation

npm install --save-dev babel-plugin-transform-catch-scoping

Usage

Add to your Babel configuration:

{
  "plugins": ["babel-plugin-transform-catch-scoping"]
}

Recommended Setup for IE8 Compatibility

This plugin should be placed last in the plugins array so that it can detect all variable names generated by preceding plugins and avoid conflicts:

{
  "plugins": [
    "babel-plugin-anonymize-function-expression",
    "@babel/plugin-transform-jscript",
    "babel-plugin-transform-catch-scoping"
  ]
}

License

MIT