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exception_handling.md

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Exception Handling

  • The try statement lets you test a block of code for errors.
  • The catch statement lets you handle the error.
  • The throw statement lets you create custom errors.
  • The finally statement lets you execute code, after try and catch, regardless of the result.

When executing JavaScript code, different errors can occur.

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
try {
    adddlert("Welcome guest!");
}
catch(err) {
    document.getElementById("demo").innerHTML = err.message;
}
</script>
</body>
</html>

Output:

adddlert is not defined

JavaScript try and catch

  • The try statement allows you to define a block of code to be tested for errors while it is being executed.
  • The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs:

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

JavaScript Throws Errors

When an error occurs, JavaScript will normally stop and generate an error message.
The technical term for this is: JavaScript will throw an exception (throw an error).
JavaScript will actually create an Error object with two properties: name and message.

The throw Statement:

The throw statement allows you to create a custom error.
Technically you can throw an exception (throw an error).
The exception can be a JavaScript String, a Number, a Boolean or an Object:

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

If you use throw together with try and catch, you can control program flow and generate custom error messages.

The finally Statement

The finally statement lets you execute code, after try and catch, regardless of the result:

try {
    Block of code to try
}
catch(err) {
    Block of code to handle errors
}
finally {
    Block of code to be executed regardless of the try / catch result
}

Example:

<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
try {
    adddlert("Welcome guest!");
}
catch(err) {
    document.getElementById("demo").innerHTML = err.message;
}
finally{
document.write("finally");
}
</script>
</body>
</html>

Output:

adddlert is not defined
finally

Error Name Values

Six different values can be returned by the error name property:

EvalError -An error has occurred in the eval() function RangeError -A number "out of range" has occurred ReferenceError -An illegal reference has occurred SyntaxError -A syntax error has occurred TypeError -A type error has occurred URIError -An error in encodeURI() has occurred

Eval Error

An EvalError indicates an error in the eval() function.

Range Error

A RangeError is thrown if you use a number that is outside the range of 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

A ReferenceError is thrown if you use (reference) a variable that has not been declared:

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

A SyntaxError is thrown if you try to evaluate code with a syntax error.

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

A TypeError is thrown if you use a value that is outside the range of expected 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

A URIError is thrown if you use illegal characters in a URI function:

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