Learn JS #32

JavaScript Errors


Throw, and Try…Catch…Finally

The try statement defines a code block to run (to try).

The catch statement defines a code block to handle any error.

The finally statement defines a code block to run regardless of the result.

The throw statement defines a custom error.


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

In this example we misspelled “alert” as “adddlert” to deliberately produce an error:

<p id="demo"></p>

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

JavaScript catches adddlert as an error, and executes the catch code to handle it.


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.


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:

<!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() {
  const message = document.getElementById("p01");
  message.innerHTML = "";
  let x = document.getElementById("demo").value;
  try {
    if(x == ""throw "empty";
    if(isNaN(x)) throw "not a number";
    x = Number(x);
    if(x < 5throw "too low";
    if(x > 10throw "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">

You can read more about forms validation in a later chapter of this tutorial.


The finally Statement

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

Syntax

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

function myFunction() {
  const message = document.getElementById("p01");
  message.innerHTML = "";
  let x = document.getElementById("demo").value;
  try {
    if(x == ""throw "is empty";
    if(isNaN(x)) throw "is not a number";
    x = Number(x);
    if(x > 10throw "is too high";
    if(x < 5throw "is too low";
  }
  catch(err) {
    message.innerHTML = "Error: " + err + ".";
  }
  finally {
    document.getElementById("demo").value = "";
  }
}

The Error Object

JavaScript has a built in error object that provides error information when an error occurs.

The error object provides two useful properties: name and message.


Error Object Properties

Property Description
name Sets or returns an error name
message Sets or returns an error message (a string)

Error Name Values

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

Error Name Description
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

The six different values are described below.


Eval Error

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

Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.


Range Error

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.

Example

let num = 1;
try {
  num.toPrecision(500);   // A number cannot have 500 significant digits
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Reference Error

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

Example

let x = 5;
try {
  x = y + 1;   // y cannot be used (referenced)
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Syntax Error

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

Example

try {
  eval("alert('Hello)");   // Missing ' will produce an error
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Type Error

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

Example

let num = 1;
try {
  num.toUpperCase();   // You cannot convert a number to upper case
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

URI (Uniform Resource Identifier) Error

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

Example

try {
  decodeURI("%%%");   // You cannot URI decode percent signs
}
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)

Do not use these properties in public web sites. They will not work in all browsers.

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