Exception Handling

Exception handling is a process of handling unusual circumstances during runtime of a program. Sometimes in these circumstances, there is a need to execute an alternate set of instructions which changes the normal flow of the program.

In exception handling, there are four prominent statements

The try statement
The catch statement
The throw statement
The finally statement

JavaScript try and catch

The try statement tests a block of code for errors during the execution of code.
The catch statement also has a block of code which assumes control immediately after the try block throws an error.

The JavaScript statements try and catch come in pairs:

try {
    Block of code to try
}
catch(err) {
    Block of code to handle errors
}


    
        <script>
/*Javascript program to use exception handling and prevent the calculation of roots of a quadratic equation if root are complex
Input
enter the equation in the form of 1 2 1 if equation is x^2 + 2x + 1:1 1 6
Output
Caught exception: roots are imaginary, so calculation stopped
*/
var i,t,r1,r2;
var k=new Array();
function quad(t,k)
{
 //if less than zero, throws exception
 if(t<0)
  throw "roots are imaginary, so calculation stopped";
 else
 {
  //calculates the roots of the equation and display the roots
  document.write("roots are:");
  var r1=(-1*k[1]+ Math.sqrt(t))/2*k[0];
  var r2=(-1*k[1]- Math.sqrt(t))/2*k[0];
  document.write(r1);
  document.write("</br>");
  document.write(r2);
 }
}
var msg="enter the equation in the form of 1 2 1 if equation is x^2 + 2x + 1:";
//takes the values of coefficients of quadratic equation
for(i=0;i<3;i++)
	k[i]=prompt(msg);
//if b^2-4ac is less than 0, then roots are imaginary
//calculate b^2-4ac
t=Math.pow(k[1],2)-4*k[0]*k[2];
try
{
 quad(t,k);
}
catch(err) {
        document.write(err);
    }
</script>
        Try

The throw Statement:

The throw statement allows us to create a customized error based on our requirement. This statement can return anything from a string to a boolean or an object.

throw "Too big"; // throw a text
throw 500; // throw a number


    
        <script>
/*JavaScript program to use Exception handling and catch divided by zero, divided by power of 2 exceptions
Input
enter divident:9
enter divisor:2
Output
Caught exception: shouldnt be divided by zero or a power of 2
*/
function check(a,b)
{
   //converts the divisor-1 value in to binary format, as 2^n-1 contains only 1''s in its binary value
   var c=b-1;
   var count=0;
   var converted = new Array();
    while(c>=1) {
      converted.unshift(c%2);
      c = Math.floor(c/2);
    }
 for(i=0;i<converted.length;i++)
 {
    if(converted[i]==0)
       var count=1;
 }
  if(b==0 || count==0)
   {
    try 
    {
     //throws the exception
     throw "shouldnt be divided by zero or a power of 2";
    }
    //catches the exception
    catch (err) 
    {
      document.write(err);
    }
   }
 else
 {
  //displays the value if no exception occurs
  var value=a/b;
  document.write(value);
 }

}
var msg="enter divident:";
a = prompt(msg);
var msg="enter divisor:";
b = prompt(msg);
check(a,b);

</script>
        Try

finally
The finally statement executes a block of code after the execution of the try/catch statement. This block of code executes regardless of what occurs during the execution of the try/catch statement.

Error names

Eval Error: Throws this error when there is an error in the eval statement.
Range Error: Throws this error when the number used is present outside the range of accepted legal values
Reference Error: Throws a reference error when there is a reference to an undeclared variable.
Syntax error: Throws a syntax error if the syntax of the code is not proper during runtime.
Type Error: Throws this error when the type of the variable is outside the range of accepted value types.
URI Error: Throws this error if we use illegal or invalid characters in the URI.

Eval Error

Throws this error when there is an error in the eval statement.

Range Error

Throws this error when the number used is present outside the range of accepted legal values

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo">
<script>
var num = 1;
try {
    num.toPrecision(500);
}
catch(err) {
    document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>

Output:

RangeError

Reference Error

Throws a reference error when there is a reference to an undeclared variable.

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
var x;
try {
    x = z + 1;
}
catch(err) {
    document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>

Output:

ReferenceError

Syntax Error

Throws a syntax error if the syntax of the code is not proper during runtime.

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
try {
    eval("alert('Hello)");
}
catch(err) {
    document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>

Output:

SyntaxError

Type Error

Throws this error when the type of the variable is outside the range of accepted value types.

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
var num = 1;
try {
    num.toUpperCase();
}
catch(err) {
    document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>

Output:

TypeError

URI Error

Throws this error if we use illegal or invalid characters in the URI.

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
try {
    decodeURI("%");
}
catch(err) {
    document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>

Output:

URIError

Exception Handling

JavaScript try and catch

The throw Statement:

finally The finally statement executes a block of code after the execution of the try/catch statement. This block of code executes regardless of what occurs during the execution of the try/catch statement.

Error names

Eval Error

Range Error

Reference Error

Syntax Error

Type Error

URI Error

finally
The finally statement executes a block of code after the execution of the try/catch statement. This block of code executes regardless of what occurs during the execution of the try/catch statement.