Javascript - Exception Handling with the Error object

About

This page is about exception in Javascript

Exceptions can be caught:

in a try, catch, throw, finally statement.
with the catch function with a promise. See catch

Example

With the error object

¹⁾

try {
  throw new Error('Badddd')
} catch (e) {
  if (e instanceof Error) {
    // you should use `console.error`
    console.log(e.name + ': ' + e.message + ". Stack Trace" + e.stack)
  }
}

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With a string

You can also throw a string. You don't get extra information such as the stack trace but it's possible.

try {
  throw 'Badddd'
} catch (e) {
    console.log(e);
}

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Syntax

The interface of an error is

interface Error {
    name: string;
    message: string;
    stack?: string;
}

How to create another Exception Type?

export class SessionError extends Error {
}
throw new SessionError('Yolo');

Management

Pause

type (instanceof)

Javascript - instanceof An error is an object, to discover the instance, just output the prototype name

try {
	// an error occured
} catch (e){
    console.log("The instance of the error is "+e.__proto__.toString()); // To get the instance of name
    // if e.__proto__.toString() ==> SyntaxError
    if (e instance SyntaxError) {
       //... Do whatever
    }
}

Variable scope

The try…catch binds a caught exception to a variable that is scoped just to the catch block (not on the function level). This is the only scope exception.

var foo = "initial value";
    
try {
	throw "exception";
} catch (foo) { //  < -- The local variable foo is a caught exception and will not affect the local variable because its scope is limited to the try catch block
	foo = "exception"; // <-- This change is only for the try block
}

console.log("The foo variable was not polluted when declared as a caught exception\n"+
  "foo will still have its initial value");
console.log(foo);

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Caugth with Google Analytics

See https://developers.google.com/analytics/devguides/collection/analyticsjs/exceptions

¹⁾

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error