When we develop something, we often need our own error classes to reflect specific things that may go wrong in our tasks. For errors in network operations we may need HttpError, for database operations DbError, for searching operations NotFoundError and so on.
Our errors should support basic error properties like message, name and, preferably, stack. But they also may have other properties of their own, e.g. HttpError objects may have statusCode property with a value like 404 or 403 or 500.
JavaScript allows to use throw with any argument, so technically our custom error classes don’t need to inherit from Error. But if we inherit, then it becomes possible to use obj instanceof Error to identify error objects. So it’s better to inherit from it.
As we build our application, our own errors naturally form a hierarchy, for instance HttpTimeoutError may inherit from HttpError, and so on.
As an example, let’s consider a function readUser(json) that should read JSON with user data. Here’s an example of how a valid json may look:
let json = `{ "name": "John", "age": 30 }`;
Internally, we’ll use JSON.parse. If it receives malformed json, then it throws SyntaxError.
But even if json is syntactically correct, that doesn’t mean that it’s a valid user, right? It may miss the necessary data. For instance, if may not have name and age properties that are essential for our users.
Our function readUser(json) will not only read JSON, but check (“validate”) the data. If there are no required fields, or the format is wrong, then that’s an error. And that’s not a SyntaxError, because the data is syntactically correct, but another kind of error. We’ll call it ValidationError and create a class for it. An error of that kind should also carry the information about the offending field.
Our ValidationError class should inherit from the built-in Error class.
That class is built-in, but we should have its approximate code before our eyes, to understand what we’re extending.
// The "pseudocode" for the built-in Error class defined by JavaScript itself
class Error {
constructor(message) {
this.message = message;
this.name = "Error"; // (different names for different built-in error classes)
this.stack = <nested calls>; // non-standard, but most environments support it
}
}
Now let’s go on and inherit ValidationError from it:
class ValidationError extends Error {
constructor(message) {
super(message); // (1)
this.name = "ValidationError"; // (2)
}
}
function test() {
throw new ValidationError("Whoops!");
}
try {
test();
} catch(err) {
alert(err.message); // Whoops!
alert(err.name); // ValidationError
alert(err.stack); // a list of nested calls with line numbers for each
}
Please take a look at the constructor:
super in the child constructor, so that’s obligatory. The parent constructor sets the message property.name property to "Error", so in the line (2) we reset it to the right value.Let’s try to use it in readUser(json):
class ValidationError extends Error {
constructor(message) {
super(message);
this.name = "ValidationError";
}
}
// Usage
function readUser(json) {
let user = JSON.parse(json);
if (!user.age) {
throw new ValidationError("No field: age");
}
if (!user.name) {
throw new ValidationError("No field: name");
}
return user;
}
// Working example with try..catch
try {
let user = readUser('{ "age": 25 }');
} catch (err) {
if (err instanceof ValidationError) {
alert("Invalid data: " + err.message); // Invalid data: No field: name
} else if (err instanceof SyntaxError) { // (*)
alert("JSON Syntax Error: " + err.message);
} else {
throw err; // unknown error, rethrow it (**)
}
}
The try..catch block in the code above handles both our ValidationError and the built-in SyntaxError from JSON.parse.
Please take a look at how we use instanceof to check for the specific error type in the line (*).
We could also look at err.name, like this:
// ...
// instead of (err instanceof SyntaxError)
} else if (err.name == "SyntaxError") { // (*)
// ...
The instanceof version is much better, because in the future we are going to extend ValidationError, make subtypes of it, like PropertyRequiredError. And instanceof check will continue to work for new inheriting classes. So that’s future-proof.
Also it’s important that if catch meets an unknown error, then it rethrows it in the line (**). The catch only knows how to handle validation and syntax errors, other kinds (due to a typo in the code or such) should fall through.
The ValidationError class is very generic. Many things may go wrong. The property may be absent or it may be in a wrong format (like a string value for age). Let’s make a more concrete class PropertyRequiredError, exactly for absent properties. It will carry additional information about the property that’s missing.
class ValidationError extends Error {
constructor(message) {
super(message);
this.name = "ValidationError";
}
}
class PropertyRequiredError extends ValidationError {
constructor(property) {
super("No property: " + property);
this.name = "PropertyRequiredError";
this.property = property;
}
}
// Usage
function readUser(json) {
let user = JSON.parse(json);
if (!user.age) {
throw new PropertyRequiredError("age");
}
if (!user.name) {
throw new PropertyRequiredError("name");
}
return user;
}
// Working example with try..catch
try {
let user = readUser('{ "age": 25 }');
} catch (err) {
if (err instanceof ValidationError) {
alert("Invalid data: " + err.message); // Invalid data: No property: name
alert(err.name); // PropertyRequiredError
alert(err.property); // name
} else if (err instanceof SyntaxError) {
alert("JSON Syntax Error: " + err.message);
} else {
throw err; // unknown error, rethrow it
}
}
The new class PropertyRequiredError is easy to use: we only need to pass the property name: new PropertyRequiredError(property). The human-readable message is generated by the constructor.
Please note that this.name in PropertyRequiredError constructor is again assigned manually. That may become a bit tedious – to assign this.name = <class name> when creating each custom error. But there’s a way out. We can make our own “basic error” class that removes this burden from our shoulders by using this.constructor.name for this.name in the constructor. And then inherit from it.
Let’s call it MyError.
Here’s the code with MyError and other custom error classes, simplified:
class MyError extends Error {
constructor(message) {
super(message);
this.name = this.constructor.name;
}
}
class ValidationError extends MyError { }
class PropertyRequiredError extends ValidationError {
constructor(property) {
super("No property: " + property);
this.property = property;
}
}
// name is correct
alert( new PropertyRequiredError("field").name ); // PropertyRequiredError
Now custom errors are much shorter, especially ValidationError, as we got rid of the "this.name = ..." line in the constructor.