How to assert that a function does not raise an exception

Question

QUnit has an assertion for testing that a function raises an exception (QUnit/raises). Is it possible -- using QUnit -- to assert that a function does not raise an exception.

I realize that it is possible to test it like in the following code:

try {
    theTest();
    ok(true);
} catch (e) {
    ok(false, "Expected to succeed");
}

But I think it ought to be possible using QUnit. Any clues?

check this http://stackoverflow.com/questions/10190392/asserting-that-a-function-throws-exceptions-with-qunit it can help — hanane, Mar 30 '14 at 20:44

Samuel Rossille · Answer 1 · 2012-03-22T12:48:26.460

18

There is no such method in qunit

However, if you just write the following code which is much shorter, you will obtain the same result with additionnal benefits

theTest();
ok(true, "My function does not crash");

1/ If the code of a test raises an exception, qunit will mark the test as failed.

2/ If you check the "no try/catch" checkbox, you will be able to debug where the exception was thrown, which is not the case with your try/catch

edited Mar 22 '12 at 12:48

answered Mar 22 '12 at 12:42

Samuel Rossille

18,940
18
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Of course! That's pretty simple. And apparently a bit too simple for me to figure out. Thanks for answering :) – anissen Mar 22 '12 at 12:51
3

As it turns out, this was a bit too simple. If the function raises an exception the test fails *without* a failure message. If the function do not raise an exception the test succeeds as expected. The message in the ok()-assertion is never used. – anissen Mar 23 '12 at 13:35
Note that you should probably also add a call to expect() inside your test() function so that the number of assertions can also be verified that way. – Timo Tijhof Apr 14 '12 at 06:35
the ok() assertion is required, as a test without assertions will fail in qunit – Armand Feb 14 '13 at 12:10

score 2 · Answer 2 · answered Dec 05 '13 at 12:15

I had the same issue as you mentioned in the comment whereby my test which tests no Error is thrown would stop "badly" showing a badly formatted Died on test #1 message without any useful information.

I ended up using a mix of both; raises() for one test and try/catch for the other.

I used raises() for the test which tests that an Error is thrown, similar to this:

test("When myFunction() is called with a invalid instance Then Error is thrown", function () {
    // Arrange
    var testInstance = {};

    // Act
    raises(function() {
        myFunction(testInstance);
    }, Error, "myFunction() should throw an Error");

    // Assert
    // raises() does assertion
});

If the above throws an Error all is fine and if not a nice formatted message is displayed, similar to this:

myFunction() should throw Error
Result: No exception was thrown.

I then used try/catch for the tests which have to ensure no Error is thrown, similar to this:

test("When myFunction() is called with a valid instance Then no Error is thrown", function () {
    // Arrange
    var testInstance = new myObject();
    var result;

    // Act
    try {
        myFunction(testInstance);
        result = true;
    } catch(error) {
        result = false;
    }

    // Assert
    ok(result, "myFunction() should not throw an Error"); 
});

If the above throws no Error all is fine and if an Error is thrown a nice formatted message is displayed, similar to this:

myFunction() should not throw an Error
Source: ...

The above is based on the assumption that you are testing managed custom thrown errors and your `myFunction()` does do an intentional `throw new Error(...)` given a certain invalid condition. You should not test for unexpected errors off course. — Nope, Dec 05 '13 at 12:24

How to assert that a function does not raise an exception

2 Answers2