How to gracefully handle errors in JavaScript?

Published October 3, 2020

Errors are something that happens unexpectedly during the execution of a program. Handling errors is crucial for better user experience and better management of an application.

In JavaScript, Errors can happen quite often due to its nature itself.

The most common causes of Errors in JavaScript are from:

  • Syntax errors
  • Errors when accessing DOM
  • Error caused by User while the application is running etc.

You may have seen an error like this,

Error

If these errors are not handled properly, it may block the other parts of code to execute and finally make the application worthless and leaves the user in bad experience too.

We cannot make our application error-prone but we can handle it gracefully.

How to handle errors in JavaScript?

In JavaScript, Errors can be handled using the try...catch block.

// try catch block
try {
  /*
    Run your unguaranteed code here
  */
} catch (error) {
  /*
    Catch any errors from running the above code 
    and deal with it here
  */
}

For example, let's say we have a function which changes the background color of an element like this,

/* 
    Function: 
    Which changes the background color of
    any element passed to it.
*/
function changeBackgroundColor(element, color) {
  element.style.backgroundColor = color;
}

But there is a slight problem with the above function. What if we executed changeBackgroundColor() and passed an incorrect reference for the element argument like this,

/* 
    Function: 
    Which changes the background color of
    any element passed to it.
*/
function changeBackgroundColor(element, color) {
  element.style.backgroundColor = color;
}

// function
changeBackgroundColor(123, "red");

We need the element argument to be a valid reference for a DOM element otherwise it wouldn't work. So then it throws an Error saying Uncaught TypeError and blocks the execution of the entire program.

Now let's wrap this function execution in a try...catch block like this,

/* 
    Function: 
    Which changes the background color of
    any element passed to it.
*/
function changeBackgroundColor(element, color) {
  element.style.backgroundColor = color;
}

/*
    Calling Function inside
    try...catch block
*/
try {
  // function
  changeBackgroundColor(123, "red");
} catch (e) {
  console.log(e.message); // Cannot set property 'backgroundColor' of undefined
  /*
        Contents of Error object:

        {
            message: "Cannot set property 'backgroundColor' of undefined"
            stack: "TypeError: Cannot set property 'backgroundColor' of undefined...    
        }
    */
}

The above catch(e) block receives the error object (here e) happened while running the code on the try{} block.

We can gracefully handle the error in our catch() block here.

See this example live in JSBin.

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