How to augment error objects

How to augment error objects

Error object properties

Error objects have both "own" properties and properties inherited from Error.prototype; some properties are normal data properties while others are virtual, see:

Overwriting .fileName, .lineNumber, and .stack

Duktape 1.4.0 and after

These properties are provided by an accessor (getter/setter) inherited from Error.prototype. The getter provides the requested result (e.g. fileName) based on an internal _Tracedata property which is added to an error when the error is created.

Starting from Duktape 1.4.0 you can use a simple property write to override .fileName, .lineNumber, and .stack:

var err = new Error('aiee');
err.fileName = 'dummy.js';
print(err.fileName);  // -> dummy.js

Equivalently in C:

duk_push_error_object(ctx, DUK_ERR_RANGE_ERROR, "invalid arg: %d", myarg);
duk_push_string(ctx, "dummy.c");
duk_put_prop_string(ctx, -2, "fileName");

Before Duktape 1.4.0

Before Duktape 1.4.0 a simple property write does not work because the inherited property is accessor which "captures" the write attempt and ignores it:

// Duktape 1.3.0
var err = new Error('aiee');
err.fileName = 'dummy.js';
print(err.fileName);  // not changed

You can still override these properties using Object.defineProperty(). This works because the inherited accessor is configurable which allows an overriding "own" property to be added to the error:

var err = new Error('aiee');
Object.defineProperty(err, 'fileName', {
    value: 'dummy.js',
    writable: true,
    enumerable: false,
    configurable: true
print(err.fileName);  // dummy.js

In C code you can use duk_def_prop() similarly:

duk_push_string(ctx, "fileName");
duk_push_string(ctx, "dummy.js");
duk_def_prop(ctx, idx_err, DUK_DEFPROP_HAVE_VALUE |
                           DUK_DEFPROP_HAVE_ENUMERABLE | /*not enumerable */

Guaranteeing Duktape 1.4.0 behavior

There's a polyfill to provide Duktape 1.4.0 behavior for previous versions:

Last edited by Sami Vaarala, 2017-08-11 19:52:01