Ensuring Exceptional Behavior: Writing Jasmine Tests for Thrown Errors
2024-07-27
- When a function encounters an unexpected condition, it can throw an
Errorobject. This object contains information about the error, including a message and a stack trace. - In Node.js, errors are typically handled using
try...catchblocks:
try {
// Code that might throw an error
} catch (error) {
// Handle the error here (e.g., log it, display a message to the user)
}
Testing for Thrown Errors with Jasmine
Jasmine provides a matcher called toThrow to verify that a function throws an error when executed. Here's how it works:
Wrap the Code in a Function:
- Jasmine's
expectfunction needs to evaluate the code that might throw an error within a function context. This ensures the error is thrown during the test execution, not before the expectation is set.
expect(function() { // Code that might throw an error }).toThrow();- Jasmine's
(Optional) Match the Error Type:
- You can optionally specify the type of error you expect to be thrown:
expect(function() { throw new TypeError("This is a type error"); }).toThrow(TypeError);- If the thrown error's type doesn't match the provided type, the test will fail.
- To ensure the thrown error has a specific message:
expect(function() { throw new Error("This is a specific error message"); }).toThrow("This is a specific error message");- The test will fail if the message doesn't match exactly.
Example Test Case:
describe("MyFunction", function() {
it("should throw an error when called with a non-number argument", function() {
expect(function() {
myFunction("hello"); // This will throw a TypeError
}).toThrow(TypeError);
});
it("should throw an error with a specific message", function() {
expect(function() {
throw new Error("This is a specific error message");
}).toThrow("This is a specific error message");
});
});
Key Points:
- By wrapping the code in a function, you allow the error to be thrown within the test context.
- Matching the error type and message makes your tests more specific and ensures they're testing the expected behavior.
const expect = require('jasmine').expect; // Assuming Jasmine is installed
function myFunction(arg) {
if (typeof arg !== 'number') {
throw new TypeError('Argument must be a number');
}
// Do something with the number argument
}
describe("MyFunction", function() {
it("should throw an error when called with a non-number argument (Node.js)", function() {
expect(function() {
myFunction("hello"); // This will throw a TypeError
}).toThrow(TypeError);
});
it("should throw an error with a specific message (Node.js)", function() {
expect(function() {
throw new Error("This is a specific error message");
}).toThrow("This is a specific error message");
});
});
Browser-Based Example (using a testing framework like Jasmine for browsers):
<!DOCTYPE html>
<html>
<head>
<title>Jasmine Test Example</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jasmine/4.16.8/jasmine.min.js"></script> <script src="your_test_file.js"></script> </head>
<body>
<script>
// Run Jasmine tests
const jasmineEnv = jasmine.createEnv();
jasmineEnv.execute();
</script>
</body>
</html>
your_test_file.js:
function myFunction(arg) {
if (typeof arg !== 'number') {
throw new TypeError('Argument must be a number');
}
// Do something with the number argument
}
describe("MyFunction", function() {
it("should throw an error when called with a non-number argument (Browser)", function() {
expect(function() {
myFunction("hello"); // This will throw a TypeError
}).toThrow(TypeError);
});
it("should throw an error with a specific message (Browser)", function() {
expect(function() {
throw new Error("This is a specific error message");
}).toThrow("This is a specific error message");
});
});
Remember:
- For Node.js, you'll need to install Jasmine using
npm install jasmine. - In the browser example, replace
"your_test_file.js"with the path to your Jasmine test script. - Adapt these examples to your specific testing environment and function under test.
- If your code interacts with external functions or libraries that might throw errors, you can leverage Jasmine spies to verify if the error was propagated correctly.
- Here's how:
describe("MyFunction", function() {
let externalFunctionSpy;
beforeEach(function() {
externalFunctionSpy = jasmine.createSpy('externalFunction');
externalFunctionSpy.and.throwError(new Error('External error'));
});
it("should propagate the error thrown by external function", function() {
expect(function() {
myFunction(externalFunctionSpy); // This will throw the error
}).toThrow(Error);
});
});
Checking for Specific Error Properties:
- In some cases, you might want to ensure the thrown error has specific properties beyond its type and message. You can access these properties within the
catchblock:
it("should throw an error with a specific property", function() {
expect(function() {
throw new Error("This is an error")
}).toThrow(function() {
const thrownError = this; // Access the thrown error within the function
return thrownError.message === "This is an error" && thrownError.code === 123;
});
});
Using Promises with catch (Async/Await):
- For asynchronous code using Promises or
async/await, you can chain acatchblock to handle potential errors:
it("should catch an error in an asynchronous function", async function() {
try {
await myAsyncFunction();
} catch (error) {
expect(error).toBeInstanceOf(Error); // Or more specific checks
}
});
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