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Common JS Errors

2 min read

What are some examples of common JavaScript errors?

As you’ve been programming in JavaScript, you’ve inevitably run into error messages. Understanding common error messages will help you debug more effectively and develop into a stronger programmer. The four common types of error messages are SyntaxError, ReferenceError, TypeError, and RangeError.

A SyntaxError happens when you write something incorrectly in your code, like missing a parenthesis, or a bracket. Think of it like a grammar mistake in a sentence. Here is a common mistake developers make when creating arrays:

const arr = ["Beau", "Quincy" "Tom"]

Each array element needs to be separated by a comma otherwise it will result in an error message.

The second common JavaScript error is a ReferenceError. There are several types of ReferenceErrors, triggered in different ways. The first type of ReferenceError would be not defined variables.

console.log(price);

In this example, we are trying to log the price variable to the console but it hasn’t been defined. This is will result in a ReferenceError.

Another example of a ReferenceError is trying to access a variable, declared with let or const, before it has been defined:

console.log(b);
const b = 50;

The example above will result in a Cannot access 'b' before initialization error.

The third common error would be a TypeError. These errors occur when you try to perform an operation on the wrong type. Here is an example of trying to use the map method on an object:

const developerObj = {
  name: "Jessica",
  country: "USA",
  isEmployed: true
};
 
developerObj.map()

This example will result in a developerObj.map is not a function error because the map method is used for arrays and not objects.

The last common error we will look at is the RangeError.

A RangeError happens when your code tries to use a value that’s outside the range of what JavaScript can handle. Here is an example of trying to assign an invalid index to the length of the array:

const arr = [];
arr.length = -1;

Since -1 is not a valid index used for arrays, this will result in a RangeError.

As you continue to program in JavaScript, just be aware of these different types of errors you will probably encounter and why they are happening.

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