Organize Code

Execption

JavaScript

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Try Catch

ECMAScript 5
function divide(a, b) {
  try {
    if (b == 0)
      throw new Error("divided by zero");
    
    console.log(a/b);
  } catch(e) {
    console.log("Cannot divide by zero");
  }
}

Try/Catch blocks are used for handling exceptions and errors that can happen in runtime.

Put the risky part of the code inside the try block and specify what to do if an error happens inside catch block.

 

Catch Specific Exception

ECMAScript 5
function divide(a, b) {
  try {
    if (b == 0)
      throw new Error("divided by zero");
    
    console.log(a/b);
  } catch(e) {
    if (e instanceof EvalError) {
      console.log(e.name + ': ' + e.message);
    } else if (e instanceof Error) {
      console.log("Cannot divide by zero");    
    } else {
      console.log("Unknown Error");
    }
  }
}

JavaScript is not a type-safe language, so in order to catch a specific exception we need to use if statements.

 

Caught Exception object

ECMAScript 5
function divide(a, b) {
  try {
    if (b == 0)
      throw new Error("divided by zero");
    
    console.log(a/b);
  } catch(e) {
    console.log(e.name);    // Error
    console.log(e.message); // divided by zero
  }
}

In JavaScript, Exceptions can be of any type.

Natively JavaScript itself throws Errors of type Error.

Error object has these properties on its prototype:

  • name - Error name
  • message - Error message
 

Finally

ECMAScript 5
function divide(a, b) {
  try {
    if (b == 0)
      throw new Error("divided by zero");
    
    console.log(a/b);
  } catch(e) {
    console.log("Cannot divide by zero");
  } finally {
    console.log("divide function executed");
  }
}

The code inside the finally block will always be executed after the Try/Catch block.

Even if you have a return statement inside a try or catch blocks, the finally block will be run before exiting the function.

 

Throw

ECMAScript 5
function divide(a, b) {
  try {
    throw new Error("Error for no reason");
  } catch(e) {
    console.log("Executed all the time");
  }
}

Error objects are a great way of organizing your code runtime errors.

If you're writing a Library or any reusable code, you want to throw different Errors when different things go wrong, so that the users of your library can do different actions in each case.

Thrown Errors bubble up in the call stack until they are caught. If it is not caught, It will be caught by the JavaScript engine.

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