In the previous tutorial, you learned to handle exceptions using JavaScript try..catch statement. The try and catch statements handle exceptions in a standard way which is provided by JavaScript. However, you can use the throw statement to pass user-defined exceptions.

In JavaScript, the throw statement handles user-defined exceptions. For example, if a certain number is divided by 0, and if you need to consider Infinity as an exception, you can use the throw statement to handle that exception.

JavaScript throw statement

The syntax of throw statement is:

throw expression;

Here, expression specifies the value of the exception.

For example,

const number = 5;
throw number/0; // generate an exception when divided by 0

Note: The expression can be string, boolean, number, or object value.

JavaScript throw with try…catch

The syntax of try...catch...throw is:

try {
    // body of try
    throw exception;
} 
catch(error) {
    // body of catch  
}

Note: When the throw statement is executed, it exits out of the block and goes to the catch block. And the code below the throw statement is not executed.

Example 1: try…catch…throw Example

const number = 40;
try {
    if(number > 50) {
        console.log('Success');
    }
    else {

        // user-defined throw statement
        throw new Error('The number is low');
    }

    // if throw executes, the below code does not execute
    console.log('hello');
}
catch(error) {
    console.log('An error caught'); 
    console.log('Error message: ' + error);  
}

Output

An error caught
Error message: Error: The number is low

In the above program, a condition is checked. If the number is less than 51, an error is thrown. And that error is thrown using the throw statement.

The throw statement specifies the string The number is low as an expression.

For example,

throw new ReferenceError('this is reference error');

Rethrow an Exception

You can also use throw statement inside the catch block to rethrow an exception. For example,

const number = 5;
try {
     // user-defined throw statement
     throw new Error('This is the throw');

}
catch(error) {
    console.log('An error caught');
    if( number + 8 > 10) {

        // statements to handle exceptions
         console.log('Error message: ' + error); 
        console.log('Error resolved');
    }
    else {
        // cannot handle the exception
        // rethrow the exception
        throw new Error('The value is low');
    }
}

Output

An error caught
Error message: Error: This is the throw
Error resolved

In the above program, the throw statement is used within the try block to catch an exception. And the throw statement is rethrown in the catch block which gets executed if the catch block cannot handle the exception.

Here, the catch block handles the exception and no error occurs. Hence, the throw statement is not rethrown.

If the error was not handled by the catch block, the throw statement would be rethrown with error message Uncaught Error: The value is low