Question

I have the following coffeescript code:

try
   do something
catch error
   log something
   throw error

Should I use throw new Error(error) instead of throw error?

What is the difference?

Answer 1

Same as in other languages like C# or Java:

• throw error throws the same Error object
• throw new Error(error) wraps it into a new Error object. The later is used, for example, in Java when you need to convert checked Exception into unchecked one. In JavaScript you don’t need to wrap exceptions as this would make stacktrace a bit longer and less pretty.

Edit: There’re some security implications as well. Here’s an example:

function noWrap() {
    try {
        var o = {}; o.nonexistingMethod();
    } catch (error) {
        throw error;
    }
}

function wrap() {
    try {
        var o = {}; o.nonexistingMethod();
    } catch (error) {
        throw new Error(error);
    }
}

Calling noWrap() produces the following error message:

"TypeError: Object #<Object> has no method 'nonexistingMethod'"
// with error.arguments === ['nonexistingMethod', o]

Calling wrap() produces the following error message:

"Error: TypeError: Object #<Object> has no method 'nonexistingMethod'"
//  with error.arguments === undefined

So, as you can see by using a wrapping Error object we can hide the arguments of original error. Suppose you’re writing one of the following:

• some kind of a library
• a script which will be loaded on a page that you don’t own (for example, some kind of like or tweet button)
• a script on a page that has some third-party scripts loaded (social buttons, ads, tracking code, etc.)

In all those cases listed above in order to stay secure you should wrap your Error objects. Otherwise you may accidentally leak references to your internal objects, functions and variables.

Edit 2: Regarding stacktraces. Both variants preserve them. Here’s a working example and I get the following stacktraces in Chrome:

// No wrapping:
TypeError: Object #<Object> has no method 'nonexistingMethod'
    at noWrap (http://fiddle.jshell.net/listochkin/tJzCF/show/:22:23)
    at http://fiddle.jshell.net/listochkin/tJzCF/show/:37:5
    at http://fiddle.jshell.net/js/lib/mootools-core-1.4.5-nocompat.js:3901:62
    at http://fiddle.jshell.net/js/lib/mootools-core-1.4.5-nocompat.js:3915:20

// Wrapping:
Error: TypeError: Object #<Object> has no method 'nonexistingMethod'
    at wrap (http://fiddle.jshell.net/listochkin/tJzCF/show/:32:15)
    at http://fiddle.jshell.net/listochkin/tJzCF/show/:44:5
    at http://fiddle.jshell.net/js/lib/mootools-core-1.4.5-nocompat.js:3901:62
    at http://fiddle.jshell.net/js/lib/mootools-core-1.4.5-nocompat.js:3915:20