Custom Error Constructors in JavaScript

Published in: JavaScript, UncategorizedPosted by Elijah Grey on 07/29/09

[Note to person reviewing this: You should make a JavaScript post category.]

Most of the time, the standard six native error constructors and the one generic error constructor are not specific enough for an error. What if you want your library to throw a custom SecurityError if it detects an XSS vector on a website? I made a function to create such constructors that behave the exact same way the native error constructors, like SyntaxError by using methods like Error.prototype.toString and the standard error object format. This code makes throwing a custom fake error constructor made with ErrorConstructor("SyntaxError") have the same output as a native SyntaxError in a JavaScript shell. I’ve tested the code in Firefox 3/3.5 and Opera 9.6 and it seems to work fine. Comment and say if it works in your browser too.

function ErrorConstructor(constructorName) {
  var errorConstructor = function(message, fileName, lineNumber) {
  // don't directly name this function, .name is used by Error.prototype.toString
    if (this == window) return new arguments.callee(message, fileName, lineNumber); =;
    this.message = message||"";
    this.fileName = fileName||location.href;
    if (!isNaN(+lineNumber)) this.lineNumber = +lineNumber;
    else this.lineNumber = 1;
  } = constructorName||;
  errorConstructor.prototype.toString = Error.prototype.toString;

  return errorConstructor;

Usage: ErrorConstructor([constructorName])

Note: If no constructorName is specified, the default of is used

Usage for generated error constructor: errorConstructor([message[, location[, lineNumber]])


var SecurityError = ErrorConstructor("Security Error"),
MarkupError = ErrorConstructor("(X)HTML Markup Error");
//these will both throw a SecurityError starting with "Security Error on line 83:"
var xss_error = "Possible XSS Vector\n\
 JSON XHR response parsed with eval()\n\
 Recommended fix: Parse JSON with JSON.parse";
throw new SecurityError(xss_error, "/js/searchResultsJSONloader.js", 83);
throw SecurityError(xss_error, "/js/searchResultsJSONloader.js", 83);
//these will both throw the following MarkupError:
//"(X)HTML Markup Error on line 1: Invalid DOCTYPE"
throw new MarkupError("Invalid DOCTYPE");
throw MarkupError("Invalid DOCTYPE");

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Posted By: Larry Vela on March 18th, 2010

Wow this awesome. I dont really realized that there are programmers guru other there who can develop code like this javacript.

By the way, is there another way of putting a javascript on which user browser will automatically adjust settings of css according to its browser used.

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