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.
Errors Will Happen!
When executing JavaScript code, different errors can occur.
Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things.
Example :
<p id="demo"></p>
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}</script>
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}</script>
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}
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:
Example :
throw "Too big"; // throw a textthrow 500; // throw a number
Input Validation Example
This example examines input. If the value is wrong, an exception (err) is thrown.
The exception (err) is caught by the catch statement and a custom error message is displayed:
Example :
<!DOCTYPE html>
<html>
<body>
<p>Please input a number between 5 and 10:</p>
<input id="demo" type="text">
<button type="button" onclick="myFunction()">Test Input</button>
<p id="p01"></p>
<script>
function myFunction() {
var message, x;
message = document.getElementById("p01");
message.innerHTML = "";
x = document.getElementById("demo").value;
try {
if(x == "") throw "empty";
if(isNaN(x)) throw "not a number";
x = Number(x);
if(x < 5) throw "too low";
if(x > 10) throw "too high";
}
catch(err) {
message.innerHTML = "Input is " + err;
}
}</script>
</body>
</html>
<html>
<body>
<p>Please input a number between 5 and 10:</p>
<input id="demo" type="text">
<button type="button" onclick="myFunction()">Test Input</button>
<p id="p01"></p>
<script>
function myFunction() {
var message, x;
message = document.getElementById("p01");
message.innerHTML = "";
x = document.getElementById("demo").value;
try {
if(x == "") throw "empty";
if(isNaN(x)) throw "not a number";
x = Number(x);
if(x < 5) throw "too low";
if(x > 10) throw "too high";
}
catch(err) {
message.innerHTML = "Input is " + err;
}
}</script>
</body>
</html>
HTML Validation
The code above is just an example.
Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes:
<input id="demo" type="number" min="5" max="10" step="1"
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}
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 :
function myFunction() {
var message, x;
message = document.getElementById("p01");
message.innerHTML = "";
x = document.getElementById("demo").value;
try {
if(x == "") throw "is empty";
if(isNaN(x)) throw "is not a number";
x = Number(x);
if(x > 10) throw "is too high";
if(x < 5) throw "is too low";
}
catch(err) {
message.innerHTML = "Error: " + err + ".";
}
finally {
document.getElementById("demo").value = "";
}
}
var message, x;
message = document.getElementById("p01");
message.innerHTML = "";
x = document.getElementById("demo").value;
try {
if(x == "") throw "is empty";
if(isNaN(x)) throw "is not a number";
x = Number(x);
if(x > 10) throw "is too high";
if(x < 5) throw "is too low";
}
catch(err) {
message.innerHTML = "Error: " + err + ".";
}
finally {
document.getElementById("demo").value = "";
}
}
Range Error
A RangeError is thrown if you use a number that is outside the range of legal values.
For example: You cannot set the number of significant digits of a number to 500.
var num = 1;
try {
num.toPrecision(500); // A number cannot have 500 significant digits}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
try {
num.toPrecision(500); // A number cannot have 500 significant digits}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Non-Standard Error Object Properties
Mozilla and Microsoft defines some non-standard error object properties:
fileName (Mozilla)
lineNumber (Mozilla)
columnNumber (Mozilla)
stack (Mozilla)
description (Microsoft)
number (Microsoft)
lineNumber (Mozilla)
columnNumber (Mozilla)
stack (Mozilla)
description (Microsoft)
number (Microsoft)
Do not use these properties in public web sites. They will not work in all browsers.
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