Each assertion should test one condition and have only one reason to fail or succeed. If an assertion success depends on multiple conditions it becomes difficult to understand if the test passed for the right reason. It also makes debugging more difficult when the test fails.

This rule raises an issue when the following Chai assertions are found:

• When .not and .throw are used together and at least one argument is provided to .throw. Such assertions succeed when the target either does not throw any exception, or when it throws an exception different from the one provided.
• When .not and .include are used together and an object is given to .include. Such assertions succeed when one or multiple key/values are missing.
• When .not and .property are used together and .property is given at least two arguments. Such assertions succeed when the target either doesn’t have the requested property, or when this property exists but has a different value.
• When .not and .ownPropertyDescriptor are used together and .ownPropertyDescriptor is given at least two arguments. Such assertions succeed when the target either doesn’t have the requested property descriptor, or its property descriptor is not deeply equal to the given descriptor
• When .not and .members are used together. Such assertions succeed when one or multiple members are missing.
• When .change and .by are used together. Such assertions succeed when the target either decreases or increases by the given delta
• When .not and .increase are used together. Such assertions succeed when the target either decreases or stays the same.
• When .not and .decrease are used together. Such assertions succeed when the target either increases or stays the same.
• When .not negates .by. Such assertions succeed when the target didn’t change by one specific delta among all the possible deltas.
• When .not and .finite are used together. Such assertions succeed when the target either is not a number, or is one of Nan, positive Infinity, negative Infinity.

Noncompliant Code Example

const expect = require('chai').expect;

describe("uncertain assertions", function() {
    const throwsTypeError = () => { throw new TypeError() }

    it("uses chai 'expect'", function() {
        expect(throwsTypeError).to.not.throw(ReferenceError) // Noncompliant
        expect({a: 42}).to.not.include({b: 10, c: 20});  // Noncompliant
        expect({a: 21}).to.not.have.property('b', 42); // Noncompliant
        expect({a: 21}).to.not.have.ownPropertyDescriptor('b', {   // Noncompliant
            configurable: true,
            enumerable: true,
            writable: true,
            value: 42,
        });
        expect([21, 42]).to.not.have.members([1, 2]); // Noncompliant

        var myObj = { value: 1 }
        const incThree = () => { myObj.value += 3; };
        const decThree = () => { myObj.value -= 3; };
        const doNothing = () => {};

        expect(incThree).to.change(myObj, 'value').by(3); // Noncompliant
        expect(decThree).to.change(myObj, 'value').by(3); // Noncompliant

        expect(decThree).to.not.increase(myObj, 'value'); // Noncompliant
        expect(incThree).to.not.decrease(myObj, 'value'); // Noncompliant

        expect(doNothing).to.not.increase(myObj, 'value'); // Noncompliant
        expect(doNothing).to.not.decrease(myObj, 'value'); // Noncompliant

        expect(incThree).to.increase(myObj, 'value').but.not.by(1); // Noncompliant

        let toCheck;
        expect(toCheck).to.not.be.finite; // Noncompliant
    });
});

Compliant Solution

const expect = require('chai').expect;

describe("uncertain assertions", function() {
    const throwsTypeError = () => { throw new TypeError() }

    it("uses chai 'expect'", function() {
        expect(throwsTypeError).to.throw(TypeError)
        expect({a: 42}).to.not.have.any.keys('b', 'c');
        expect({a: 21}).to.not.have.property('b');
        expect({a: 21}).to.not.have.ownPropertyDescriptor('b');
        expect([21, 42]).to.not.include(1).and.not.include(2);

        var myObj = { value: 1 }
        const incThree = () => { myObj.value += 3; };
        const decThree = () => { myObj.value -= 3; };
        const doNothing = () => {};

        expect(incThree).to.increase(myObj, 'value').by(3);
        expect(decThree).to.decrease(myObj, 'value').by(3);

        expect(decThree).to.decrease(myObj, 'value').by(3);
        expect(incThree).to.increase(myObj, 'value').by(3);

        expect(doNothing).to.not.change(myObj, 'value');

        expect(incThree).to.increase(myObj, 'value').by(3);

        let toCheck;
        // Either of the following is valid
        expect(toCheck).to.be.a('string');
        expect(toCheck).to.be.NaN;
        expect(toCheck).to.equal(Infinity);
        expect(toCheck).to.equal(-Infinity);
    });
});