Throw statements

Throw statements are used in combination with Try-Catch staements to define an user defined exceptions. When an error is caught during execution of a try block, the throw statement is executed and program flow goes to catch statements as needed.

NOTE: The code after the throw statement is not executed. If there is no catch block defined, the program terminates immediately.

Syntax for Throw statement

throw expression;

Where:

expression – It is expression which gets thrown when error occurs.

While throwing an exception an object can be referenced too. That can be a JavaScript error object such as RangeError, EvalError, SyntaxError … etc or an user defined object. A throw exception can also be re-thrown, until an exception handler is found or more debugging information is found.

Example of user-defined error object throwing error message

<!DOCTYPE html>
<html>

<body>
	<form name="myForm" onsubmit="validateForm()">
		Enter your mail ID:
		<input type="text" id="email" /><br/>
		<input type="submit" value="Submit" />
	</form>
		
	<script>	
		function MyException(value){
			this.value = value;
			this.message = "This is not a valid mail Id";
			this.toString = function(){
				return this.value + this.message;
			};
		}
		
		function validateForm() {
			try{
				var x = document.forms["myForm"]["email"].value;
				var filter=/^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i
				if ((filter.test(x))){
				alert("A message will be sent to this mail ID");		
				}
				else{
					throw new MyException(x);
				}
			}catch(err){
				if (err instanceof MyException){
				  alert("User defined exception " + err.value + " " + err.message);
				}
				else{
				   alert("Other exception " + err.value + " " + err.message);
				}
			}
		} 
	</script>
</body>

</html>

Example with multiple throw statements and JavaScript error objects

<!DOCTYPE html>

<html>
<body>
	<input type = "text" id="a" />
	<input input type = "text" id="b" />
	<button onclick="getquo()">Get Quotient</button>
	<br/><br/>
	<script>
    function getquo(){	
		var a = Number(document.getElementById("a").value);
		var b = Number(document.getElementById("b").value);		
		try{
			if(a == 0) 
				throw new RangeError("0 divided by anything is 0");
				
			else if((isNaN(a))|| (isNaN(b))) 
				throw new TypeError("Input type is not compatible for division");						
		}catch(err){			
			document.write("In catch statement " + err.name + " " + err.message);			
		}
	}
</script>
</body>

</html>

›› go to examples ››

Syntax for Throw statement

Example of user-defined error object throwing error message

Example with multiple throw statements and JavaScript error objects

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