Try this:

try {
    assert.commandWorked({
        "n" : 0,
        "writeErrors" : [
                {
                        "index" : 0,
                        "code" : 263,
                        "errmsg" : "foo"
                }
        ],
        "ok" : 1
    });
} catch (ex) {
    print(typeof ex.code);
}

Expected: number
Actual: object

The problem is _getErrorWithCode(), compiled into the shell from utils.js. That function expects the provided error object to be a command failure with ok: 0, or an object with a "writeError" field, not a "writeErrors" array.

I think _getErrorWithCode() should check for writeErrors, too. I'm not sure "writeError" is ever actually present.

This is a problem for tests that check things like "ex.code === 123". If the command failed with a writeErrors array, the exception's code is an object, not the expected number, and the check may be false incorrectly.