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In JavaScript, the try statement is used to handle errors (also called exceptions
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) that may occur during code execution - without stopping the entire program.The try statement works together with catch. Sometimes it works with finally. And sometimes it works with throw.
The try block contains the code that might throw an error. If no error occurs, the catch block is skipped.
Syntax try {// Code that may cause an error
} catch (error) {// Code to handle the error
}The catch block executes only if an error occurs in the try block. The error object provides details about what went wrong.
Syntax try {// Code that may cause an error
} catch (error) {// Code to handle the error
}
The Finally Block (Optional)The finally block executes after the try and catch blocks, whether an error occurred or not. It is commonly used for cleanup tasks (e.g., closing files, stopping loaders, etc.).
Syntax try {// Code that may cause an error
} catch (error) {// Code to handle the error
} finally {// Code that always runs, no matter what
}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).
name and message.
The throw statement allows you to create a custom error. Technically you can throw an exception (throw an error).
String, a Number, a
throw "Too big"; // throw a text throw 500; // throw a numberIf you use throw together with try and catch, you can control program flow and generate custom error messages.
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: <!DOCTYPE html> <html> <body>
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< p > Please input a number between5 and 10:</p> <input id="demo" type="text"> <button type="button" onclick="myFunction()">Test Input</button> <p id="p01"></p> <script>
function myFunction() {
const message =
document.getElementById("p01");
message.innerHTML = "";
let x =
document.getElementById("demo").value;
try {
if(x.trim() == "") 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>
The code above is just an example.
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Modern browsers will often use a combination of JavaScript and built - in HTMLvalidation, using predefined validation rules defined in HTML attributes: <input id="demo" type="number" min="5" max="10" step="1"> You can read more about forms validation in a later chapter of this tutorial.
The finally statement lets you execute code, after try and catch, regardless of the result:
Syntax try {} catch(err
) {} finally {Formula
Block of code to be executed regardless of the try / catch result}Example function myFunction() {
const message =
document.getElementById("p01");
message.innerHTML = "";
let x =
document.getElementById("demo").value;
try {
if(x.trim() == "") 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 = "";
}
}