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JavaScript Statement: try … catch … finally

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In JavaScript, the try … catch … finally statement (often used in conjunction with the throw statement) implements error handling. The statement takes e.g. the following format:


try {



catch(e) {



finally {




In the above, JavaScript will first try to execute code1. If an error occurs, e.g. the code cannot be understood or is misspelt and the (optional) catch statement is present then instead of just stopping (which would be the usual response to an error in the absence of a try statement) it moves to code2. Regardless of the try / catch result, if there is an (optional) finally statement it will then execute code3. The type of error thrown (which can be system specified or specified by the developer using the throw statement is available through e, which is the name used in the code to specify the local Error object identifying the error.


Note: the catch and finally statement components are both optional, but you typically need to include at least one of them when using a try statement.


Error objects have two intrinsic properties, the .message property which contains a description of the error and the .number property which contains the error number of the error. Note, some browser suppliers e.g. Microsoft have additional non-standard properties such as .description, which seems otherwise to be the same as .message but will not be recognised by non-Microsoft browsers (so should be avoided if users are likely to use other browsers to view the relevant webpage).


Other modern object-orientated programming languages such as Visual Basic also typically now include structured error handling like the above, but potentially with different forms of error object and with the error object potentially more simply handling errors triggered within function calls.



<!DOCTYPE html>
<html> <!-- Copyright (c) Nematrian Limited 2018 -->
table,th,tr,td {border: 1px solid black; border-collapse: collapse;}
<th>Resulting value of <code>x</code></th>
<td><code id="Example"></code></td>
<td><code id="Result"></code></td>

function crSp(n) { return '<br>' + ('&nbsp;').repeat(n) }
var x = "";
var y = 1;
try { failprocess(y) }
catch(e) { x = e.description; }
finally {
  if (x.length>0) {x = "Caught error is: " + x }
document.getElementById("Example").innerHTML =
     'var x = "";'
     + crSp(0) + 'var y = 1;'
     + crSp(0) + 'try { failprocess(y) }'
     + crSp(0) + 'catch(e) { x = e.description; }'
     + crSp(0) + 'finally {'
     + crSp(2) + 'if (x.length>0) {x = "Caught error is: " + x }'
     + crSp(0) + '}';
document.getElementById("Result").innerHTML = x;


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