Testing for errors thrown in Mocha [duplicate]

Answer 1

Adding to the top answer, if you need to invoke your function as part of the test (i.e. your function should only throw an error if certain parameters are passed), you can wrap your function call in an anonymous function, or, in ES6+, you can pass your function in an arrow function expression.

// Function invoked with parameter.
// TEST FAILS. DO NOT USE.
assert.throws(iThrowError(badParam), Error, "Error thrown"); // WRONG!

// Function invoked with parameter; wrapped in anonymous function for test.
// TEST PASSES.
assert.throws(function () { iThrowError(badParam) }, Error, "Error thrown");

// Function invoked with parameter, passed as predicate of ES6 arrow function.
// TEST PASSES.
assert.throws(() => iThrowError(badParam), Error, "Error thrown");

And, just for the sake of thoroughness, here's a more literal version:

// Explicit throw statement as parameter. (This isn't even valid JavaScript.)
// TEST SUITE WILL FAIL TO LOAD. DO NOT USE, EVER.
assert.throws(throw new Error("Error thrown"), Error, "Error thrown"); // VERY WRONG!

// Explicit throw statement wrapped in anonymous function.
// TEST PASSES.
assert.throws(function () { throw new Error("Error thrown") }, Error, "Error thrown");

// ES6 function. (You still need the brackets around the throw statement.)
// TEST PASSES.
assert.throws(() => { throw new Error("Error thrown") }, Error, "Error thrown");