PromiseWorker.jsm
function customerror(message) { this.message = message; } customerror.prototype.tomsg = function() { return { exn: 'customerror', message: this.message, }; }; // a function called by message.
... throw new customerror('meow'); } the converted message will be posted back to the main thread, and it will be converted again to error object, with frommsg function specified for the error in exceptionhandlers property.
...function customerror(message) { this.message = message; } customerror.frommsg = function(msg) { return new customerror(msg.message); }; // register a constructor.
... myworker.exceptionhandlers['customerror'] = customerror.frommsg; this is seen in a simple demo at github :: promiseworker custom errors demo - worker side setup.

Error - JavaScript
es6 custom error class versions of babel prior to 7 can handle customerror class methods, but only when they are declared with object.defineproperty().
... some browsers include the customerror constructor in the stack trace when using es2015 classes.
... class customerror extends error { constructor(foo = 'bar', ...params) { // pass remaining arguments (including vendor specific ones) to parent constructor super(...params) // maintains proper stack trace for where our error was thrown (only available on v8) if (error.capturestacktrace) { error.capturestacktrace(this, customerror) } this.name = 'customerror' // custom debugging information this.foo = foo this.date = new date() } } try { throw new customerror('baz', 'bazmessage') } catch(e) { console.error(e.name) //customerror console.error(e.foo) //baz console.error(e.message) //bazmessage console.error(e.stack) //stacktrace } es5 custom error object all browsers include the customerror constructor in the stack trace when using...
... function customerror(foo, message, filename, linenumber) { var instance = new error(message, filename, linenumber); instance.name = 'customerror'; instance.foo = foo; object.setprototypeof(instance, object.getprototypeof(this)); if (error.capturestacktrace) { error.capturestacktrace(instance, customerror); } return instance; } customerror.prototype = object.create(error.prototype, { constructor: { value: error, enumerable: false, writable: true, configurable: true } }); if (object.setprototypeof){ object.setprototypeof(customerror, error); } else { customerror.__proto__ = error; } try { throw new customerror('baz', 'bazmessage'); } catch(e){ console.error(e.name); //customerror console.error(e.foo); //baz console.error(...

<input>: The Input (Form Input) element - HTML: Hypertext Markup Language
in addition to the errors described in the table above, the validitystate interface contains the badinput, valid, and customerror boolean readonly properties.
... the validity object includes: validitystate.valuemissing validitystate.typemismatch validitystate.patternmismatch validitystate.toolong validitystate.tooshort validitystate.rangeunderflow validitystate.rangeoverflow validitystate.stepmismatch validitystate.badinput validitystate.valid validitystate.customerror for each of these boolean properties, a value of true indicates that the specified reason validation may have failed is true, with the exception of the valid property, which is true if the element's value obeys all constraints.

Client-side form validation - Learn web development
note: there are several errors that will prevent the form from being submitted, including a badinput, patternmismatch, rangeoverflow or rangeunderflow, stepmismatch, toolong or tooshort, typemismatch, valuemissing, or a customerror.

ValidityState - Web APIs
customerror read only a boolean indicating whether the element's custom validity message has been set to a non-empty string by calling the element's setcustomvalidity() method.