Merge pull request #866 from meeber/throw-doc

docs: rewrite `.throw` jsdoc

lib/chai/core/assertions.js

@@ -1501,31 +1501,113 @@ module.exports = function (chai, _) {
   Assertion.addMethod('key', assertKeys);
 
   /**
-   * ### .throw(constructor)
+   * ### .throw([errorLike], [errMsgMatcher])
    *
-   * Asserts that the target function will throw a specific error, or specific type of error
+   * Invokes the target function and asserts that an error is thrown. The
-   * (as determined using `instanceof`), optionally with a RegExp or String inclusion test
+   * `throw` assertion accepts optional arguments that can be used to perform
-   * for the error's message.
+   * additional assertions on the thrown error, as explained below.
-   * If an Error instance is provided, it asserts that the error thrown and the instance
-   * provided are the same.
-   * If an Error constructor is provided, it asserts that the error thrown is an instance
-   * of the constructor provided.
    *
-   *     var err = new ReferenceError('This is a bad function.');
+   * If no arguments are provided, `throw` invokes the function and asserts that
-   *     var fn = function () { throw err; }
+   * an error is thrown.
-   *     expect(fn).to.throw();
-   *     expect(fn).to.throw(ReferenceError);
-   *     expect(fn).to.throw(Error);
-   *     expect(fn).to.throw(/bad function/);
-   *     expect(fn).to.not.throw('good function');
-   *     expect(fn).to.throw(ReferenceError, /bad function/);
-   *     expect(fn).to.throw(err);
    *
-   * Furthermore, `throw` changes the context of the assertion (just like `property`)
+   *     var badFn = function () { throw new TypeError("Illegal salmon!"); };
-   * to the thrown error. This permits for further chainable assertions on the thrown error.
+   *     expect(badFn).to.throw();
+   *     var goodFn = function () {};
+   *     expect(goodFn).to.not.throw();
    *
-   *     expect(fn).to.throw(ReferenceError)
+   * If the first argument is an error constructor, `throw` invokes the function
-   *       .and.have.property('message').equal('This is a bad function.');
+   * and asserts that an error is thrown that's an instance of that constructor.
+   *
+   *     var badFn = function () { throw new TypeError("Illegal salmon!"); };
+   *     expect(badFn).to.throw(TypeError);
+   *     expect(badFn).to.not.throw(ReferenceError);
+   *
+   * If the first argument is an error instance, `throw` invokes the function
+   * and asserts that an error is thrown that's referentially equal (===) to
+   * that instance.
+   *
+   *     var err = new TypeError("Illegal salmon!");
+   *     var badFn = function () { throw err; };
+   *     expect(badFn).to.throw(err);
+   *     expect(badFn).to.not.throw(new TypeError("Illegal salmon!"));
+   *
+   * If the first argument is a string, `throw` invokes the function and asserts
+   * that an error is thrown with a message that contains that string.
+   *
+   *     var badFn = function () { throw new TypeError("Illegal salmon!"); };
+   *     expect(badFn).to.throw("salmon");
+   *     expect(badFn).to.not.throw("kangaroo");
+   *
+   * If the first argument is a regular expression, `throw` invokes the function
+   * and asserts that an error is thrown with a message that matches that
+   * regular expression.
+   *
+   *     var badFn = function () { throw new TypeError("Illegal salmon!"); };
+   *     expect(badFn).to.throw(/salmon/);
+   *     expect(badFn).to.not.throw(/kangaroo/);
+   *
+   * It's also possible to combine two assertions into one by providing two
+   * arguments. If the first argument is an error instance or constructor, and
+   * the second argument is a string or regular expression, `throw` invokes the
+   * function and asserts that an error is thrown that passes both assertions as
+   * described above.
+   *
+   *     var err = new TypeError("Illegal salmon!");
+   *     var badFn = function () { throw err; };
+   *     expect(badFn).to.throw(TypeError, "salmon");
+   *     expect(badFn).to.throw(TypeError, /salmon/);
+   *     expect(badFn).to.throw(err, "salmon");
+   *     expect(badFn).to.throw(err, /salmon/);
+   *     expect(badFn).to.not.throw(TypeError, "kangaroo");
+   *     expect(badFn).to.not.throw(TypeError, /kangaroo/);
+   *     expect(badFn).to.not.throw(ReferenceError, "salmon");
+   *     expect(badFn).to.not.throw(ReferenceError, /salmon/);
+   *     expect(badFn).to.not.throw(err, "kangaroo");
+   *     expect(badFn).to.not.throw(err, /kangaroo/);
+   *     expect(badFn).to.not.throw(new TypeError("Illegal salmon!"), "salmon");
+   *     expect(badFn).to.not.throw(new TypeError("Illegal salmon!"), /salmon/);
+   *
+   * Note that `throw` changes the context of the assertion to the thrown error.
+   * This allows for further chainable assertions on the thrown error.
+   *
+   *     var err = new TypeError("Illegal salmon!");
+   *     err.code = 42;
+   *     var badFn = function () { throw err; };
+   *     expect(badFn).to.throw(TypeError).and.have.property('code', 42);
+   * 
+   * Beware of some common mistakes when using the `throw` assertion. One common
+   * mistake is to accidentally invoke the function yourself instead of letting
+   * the `throw` assertion invoke the function for you. For example, when
+   * testing if a function named `fn` throws, provide `fn` instead of `fn()` as
+   * the target for the assertion:
+   *
+   *     expect(fn).to.throw();     // Good! Tests `fn` as desired
+   *     expect(fn()).to.throw();   // Bad! Tests result of `fn()`, not `fn`
+   *
+   * If you need to assert that your function `fn` throws when passed certain
+   * arguments, then wrap a call to `fn` inside of another function like so:
+   *
+   *     expect(function () { fn(42); }).to.throw();  // Function expression
+   *     expect(() => fn(42)).to.throw();             // ES6 arrow function
+   *
+   * Another common mistake is to provide an object method (or any stand-alone
+   * function that relies on `this`). Doing so is problematic because the `this`
+   * context will be lost when the function is invoked by the `throws`
+   * assertion; there's no way for it to know what `this` is supposed to be.
+   * There are two ways around this problem. One solution is to wrap the method
+   * or function call inside of another function. Another solution is to use
+   * `bind`. For example:
+   *
+   *     expect(function () { cat.meow(); }).to.throw();  // Function expression
+   *     expect(() => cat.meow()).to.throw();             // ES6 arrow function
+   *     expect(cat.meow.bind(cat)).to.throw();           // Bind
+   *
+   * Finally, it's worth mentioning that it's a best practice in JavaScript to
+   * only throw `Error` and derivatives of `Error` such as `ReferenceError`,
+   * `TypeError`, and user-defined objects that extend `Error`. No other type of
+   * value will generate a stack trace when initialized. With that said, the
+   * `throw` assertion does technically support any type of value being thrown,
+   * not just `Error` and its derivatives.
    *
    * @name throw
    * @alias throws