It is not good enough to test if an exception is raised, without checking which exception it is. Such tests will not be able to differentiate the
expected exception from an unexpected one. They should instead validate the exception message and/or type.
This rule raises an issue in the following cases:
- When an asynchronous Mocha test calls the
done() callback, without parameters, in a catch block and there is no
reference to the caught exception in this block. Either the error should be passed to done() or the exception should be checked
further.
- When Chai assertions are used to test if a function throws any exception, or an exception of type
Error without checking the
message.
- When Chai assertions are used to test if a function does not throw an exception of type
Error without checking the message.
Rule doesn’t raise an issue when assertion is negated using not, it such case the exception doesn’t need to be specific.
Noncompliant Code Example
const expect = require("chai").expect;
const fs = require("fs");
describe("exceptions are not tested properly", function() {
const funcThrows = function () { throw new TypeError('What is this type?'); };
const funcNoThrow = function () { /*noop*/ };
it("forgot to pass the error to 'done()'", function(done) {
fs.readFile("/etc/zshrc", 'utf8', function(err, data) {
try {
expect(data).to.match(/some expected string/);
} catch (e) { // Noncompliant
// Either the exception should be passed to done(e), or the exception should be tested further.
done();
}
});
});
it("does not 'expect' a specific exception", function() {
expect(funcThrows).to.throw(); // Noncompliant
// Error is not precise enough
expect(funcThrows).to.throw(Error); // Noncompliant
});
});
Compliant Solution
const expect = require("chai").expect;
const { AssertionError } = require('chai');
const fs = require("fs");
describe("exceptions are tested properly", function() {
const funcThrows = function () { throw new TypeError('What is this type?'); };
const funcNoThrow = function () { /*noop*/ };
it("forgot to pass the error to 'done()'", function(done) {
fs.readFile("/etc/zshrc", 'utf8', function(err, data) {
try {
expect(data).to.match(/some expected string/);
} catch (e) {
expect(e).to.be.an.instanceof(AssertionError);
done();
}
});
});
it("does not 'expect' a specific exception", function() {
expect(funcThrows).to.throw(TypeError);
expect(funcNoThrow).to.not.throw(Error, /My error message/);
});
});
