Disallow Assignment of the Exception Parameter (no-ex-assign)

When an error is caught using a catch block, it’s possible to accidentally (or purposely) overwrite the reference to the error. Such as:

try {
    // code
} catch (e) {
    e = 10;
}

This makes it impossible to track the error from that point on.

Rule Details

This rule’s purpose is to enforce convention. Assigning a value to the exception parameter wipes out all the valuable data contained therein and thus should be avoided. Since there is no arguments object to offer alternative access to this data, assignment of the parameter is absolutely destructive.

The following patterns are considered warnings:

try {
    // code
} catch (e) {
    e = 10;
}

The following patterns are considered okay and do not cause warnings:

try {
    // code
} catch (e) {
    var foo = 'bar';
}

Notes

Related aside: there are some interesting caveats in IE 6-8 where the exception identifier will leak into the outer scope causing some unexpected behavior. Ben Alman has a great article that explains this behavior in detail

Further Reading

• Do not assign to the exception parameter
• The “catch” with try…catch – Ben Alman

Version

This rule was introduced in ESLint 0.0.9.

Resources

• Rule source
• Documentation source