What is an uncaught TypeError in JavaScript?

TypeError is an error that is thrown when an expression or statement is evaluated, and the type of the expression or statement is incompatible with the type that is expected.

This type of error can occur in JavaScript when you try to use a value where a different type is required, such as using a number instead of a string.

In the following code example, the variable msg  is assigned the value Hello, world! but later attempts to assign the value 2 to the variable "msg" will generate a TypeError because a number is not compatible with a string.

var msg = "Hello, world!";
msg = 2;

In JavaScript, you can usually determine the type of an expression by the type of the variable that is assigned to it.

In the following code example, the variable x is assigned the value Hello, world! and the variable y is assigned the value 2.

var x = "Hello, world!";
var y = 2;

Since x is assigned a string and y is assigned a number, we can say that x is of type string and y is of type number.

We can also use the typeof operator to determine the type of an expression.

In the following code example, the typeof operator is used to determine the type of the x variable.

console.log(typeof x);

The output from this code will be string.

Now let's take a look at a more complex example. In the following code, the x variable is assigned the value of the msg variable multiplied by the value of the y variable.

var x = msg * y;

Since msg is a string and y is a number, the JavaScript interpreter will first convert the string Hello, world! to the number 12 and then multiply the two numbers.

So the value of x would be 144.

Now let's try to change the value of the y variable to something that is not a number.

var y = "Hello, world!";

Since y is not a number, the JavaScript interpreter will throw a TypeError.

There are a few ways to handle TypeError in JavaScript.

One way is to use the try...catch statement.

How to use try...catch in JavaScript

The try...catch statement allows you to catch errors that are thrown by a function.

In the following code example, the try...catch statement is used to catch the TypeError that is thrown when the y variable is not a number.

var y = "Hello, world!";
try {
    var x = msg * y;
    } catch (e) {
    console.log("TypeError: " + e.message);
}

The output from this code will be TypeError: y is not a number.

Another way to handle TypeError is to use the alert function.

The alert function will display a message to the user and then stop the execution of the JavaScript code.

In the following code example, the alert function is used to display a message when the y variable is not a number.

var y = "Hello, world!";
alert("y is not a number");

The output from this code will be y is not a number.