Why doesn't instanceof work on instances of Error subclasses under babel-node?
tl;dr If you're on Babel 6, you can use https://www.npmjs.com/package/babel-plugin-transform-builtin-extend
Extending builtin types like Array
and Error
and such has never been supported in Babel. It is perfectly valid in a real ES6 environment, but there are requirements to make it work that are very difficult to transpile in a way that is compatible with older browsers. It "worked" in Babel 5 in that it didn't throw an error, but objects instantiated from the extended subclass did not work like they were supposed to, for example:
class MyError extends Error {}
var e1 = new MyError();
var e2 = new Error();
console.log('e1', 'stack' in e1);
console.log('e2', 'stack' in e2);
results in
e1 false
e2 true
While it did not error out, the subclass does not properly get a 'stack' like errors are supposed to. Similarly, if you were to extend Array
it might behave somewhat like an array, and have array methods, but it did not behave fully like an array.
The Babel 5 documentation specifically called this out as an edge-case of classes to be aware of.
In Babel 6, classes were changed to be more spec-compliant in how subclassing is handled, and a side-effect of that is that now the above code will still not work, but it will not work in a different way than before. This has been covered in https://phabricator.babeljs.io/T3083, but I'll elaborate here on a potential solution.
To return Babel 5 subclassing behavior (which remember, is still not right or recommended), you can wrap the builtin constructor in your own temporary class, e.g.
function ExtendableBuiltin(cls){
function ExtendableBuiltin(){
cls.apply(this, arguments);
}
ExtendableBuiltin.prototype = Object.create(cls.prototype);
Object.setPrototypeOf(ExtendableBuiltin, cls);
return ExtendableBuiltin;
}
With this helper, rather than doing
class MyError extends Error {}
do
class MyError extends ExtendableBuiltin(Error) {}
In your specific case however, you have said that you are on Node 5.x. Node 5 has support for native ES6 classes without transpiling. I'd recommend you use those by dropping the es2015
preset and instead using node5
so you get native classes, among other things. In that context,
class MyError extends Error {}
will work the way you expect.
For people not on Node 4/5, or recent Chrome only, you may want to consider using something like https://www.npmjs.com/package/error. You can also explore https://www.npmjs.com/package/babel-plugin-transform-builtin-extend. The approximate
option from that is the same behavior from Babel 5. Beware that the non-approximate
behavior is definitely edge-casey and may not work in 100% of cases.
instanceof
will not work for subclassed errors if the compile target is set to "es5". I set the target to "es6" in my tsconfig.json
and instanceof
produced the right result.