Chai: expecting an error or not depending on a parameter

javascript mocha chai
Apzx picture Apzx · Sep 30, 2013 · Viewed 8.7k times · Source

I've been trying to do a text of a function that handles errors in a way that, if it is a valid error, it is thrown, but if it is not, then nothing is thrown. The problem is that i cant seem to set the parameter while using:

expect(handleError).to.throw(Error);

The ideal would be to use:

expect(handleError(validError)).to.throw(Error);

Is there any way to achieve this functionality?

code of the function:

function handleError (err) {
    if (err !== true) {
        switch (err) {
            case xxx:
            ...
        }
        throw "stop js execution";
    else {}
}

And the code of the test (not working as intended):

it("should stop Javascript execution if the parameter isnt \"true\"", function() {
    expect(handleError).to.be.a("function");
    expect(handleError(true)).to.not.throw(Error);
    expect(handleError("anything else")).to.throw(Error);
});

Answer

David Norman picture David Norman · Oct 3, 2013

The problem is that you are calling handleError, then passing the result to expect. If handleError throws, then expect never even gets called.

You need to defer calling handleError until expect is called, so that expect can see what happens when the function is called. Fortunately, this is what expect wants:

expect(function () { handleError(true); }).to.not.throw();
expect(function () { handleError("anything else") }).to.throw("stop js execution");

If you read the documentation for throw, you'll see that the associated expect should be passed a function.

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