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javascriptexceptionerror-handling
Jberry
Jberry

Reputation: 589

What's the difference between Error and Exception in Javascript?

It seems that Error and Exception are the same thing: both of them can be thrown with a throw statement and caught with a try catch block.

Upvotes: 57

Views: 20734

Answers (4)

Ziaullhaq Savanur
Ziaullhaq Savanur

Reputation: 2358

try {
    throw "throwing string";
} catch (ex) {
    console.log(ex);
}

If we run this code, we will observe that catch clause will receive whatever we throw using throw keyword. It means if we throw string we will receive string, if we throw object we will receive object, if we throw number we will receive number.

Note: So if we throw Error we will receive Error.

Hence the conclusion is whatever we throw using throw keyword will be considered as exception. If we don't catch(or handle) it then our Javascript code blows up.

Upvotes: 2

flavian
flavian

Reputation: 28511

JavaScript syntax

Errors and exceptions are syntactically synonymous in JavaScript. The language only implements the Error keyword (through window.Error). You may define custom errors, using the Error.constructor, which takes name and message as parameters.

JavaScript Error

There is also the line number sugar that can be used to trace bug occurrences inside the code. JavaScript only has Error. Whether you hear people talking about Exceptions or Errors, in JavaScript they refer to the same thing.

Browsers make a distinction: ReferenceError (when accessing a variable with a name that doesn't exist in the heap, or when you make a typo(more here.), TypeError is also a known JS error, more here.

JavaScript Exception

A known JavaScript Exception is DOM Exception 8. It occurs when you access an object that is not ready, such as an XMLHttpRequest that has not completed the request.

Implementation

When using try catch or try catch finally blocks, you will deal with both JavaScript Exception and Error. Code-wise the difference has no impact.

Behind the scenes, the browsers use the same window.Error constructor. An Exception is an Error instance with a name and message that contain "Exception".

Try: var myCustomError = new Error("myException", "An exception occurred.");. "Exception" is text in a string. More on Error here.

Convention

By convention, there is a difference between Error and Exception. An Error indicates a clear violation. A TypeError or ReferenceError means you are not following the language specs.

An Exception is thrown when you access an XMLHttpRequest response before it is complete. Error is a "you broke the law" shout and Exception is an "Almost there!" pad on the shoulder. Hope the analogy helps!

Upvotes: 57

Damian Leszczyński - Vash
Damian Leszczyński - Vash

Reputation: 30875

Based on the lecture, the Errors are thrown by JavaScript engine and exception are thrown by developer. it is only naming convention.

  • JavaScript throws Errors
  • Developers throws Exceptions

In technical aspect is the same structure (thing).

Upvotes: 25

Xaltar
Xaltar

Reputation: 1716

I think that an error is something serious that you should not try to catch An exception is something that should be caught, in so far as it is possible to handle it.

An error is when something wrong or invalid happens in the code. It can cause a memory error, it is something that should never happen and can't be treated.

Whereas an exception throws something when certain conditions are met in the code. It may not correspond to a real error.

Upvotes: 2

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