Chai Change

Assert that a change you expected to happen, happened, with this plugin for the chai assertion library. The plugin works in node and the browser, asynchronously or synchronously.

The idea of the plugin is to make your tests more robust. Rather than doing:

users.create();
expect(users.count()).to.equal(1);

instead assert that the action actually causes the expected change

expect(() => {
  users.create();
}).to.alter(users.count, { by: 1 });

This is more robust as it avoids false positives: in this example, if users.count() was already 1 and users.create() was not implemented, the first example would still pass. Using the change expectation, since there was not a change {by: 1} from the starting value, the test would correctly fail.

Installation

Node.js

chai-change is available on npm.

  $ npm install chai-change

Browser

Either install via npm, or download chai-change and save as chai-change.js. Then simply include after chai.js.

<script src="chai-change.js"></script>

Plug In

If you are using chai-change in the browser, there is nothing you need to do.

If you are using node, you just need to tell chai about the plugin:

const chai = require('chai');
 
chai.use(require('chai-change'));

Expect API

.change

Asserts that the value returned by function passed to change() changes after the function has run:

let x = 0;
 
expect(() => { x += 1; }).to.alter(() => x);
expect(() => {         }).not.to.alter(() => x);

You can pass options to be specific about the changes expected. Use the from key to enforce a starting value, a to key for and ending value, and a by key to enforce a numeric change.

expect(() => { x += 1 }).to.alter(() => x, { by: 1 });
expect(() => { x += 1 }).to.alter(() => x, { from: x });
expect(() => { x += 1 }).to.alter(() => x, { from: x, to: x + 1 });
expect(() => { x += 1 }).to.alter(() => x, { to: x + 1 });

Assert API

assert.alters

Asserts that the value returned by changeWatcher changes after the changer function has run:

let x = 0;
assert.alters(changer, changeWatcher);
 
function changer() { x += 1; }
function changeWatcher() { return x }

You can pass options to be specific about the changes expected. Use the from key to enforce a starting value, a to key for and ending value, and a by key to enforce a numeric change.

assert.alters(() => { x += 1 }, () => x, { by: 1 });
assert.alters(() => { x += 1 }, () => x, { from: x });
assert.alters(() => { x += 1 }, () => x, { from: x, to: x + 1 });
assert.alters(() => { x += 1 }, () => x, { to: x + 1 });

assert.unaltered

Asserts that the value returned by changeWatcher doesn't change after the changer has run:

let x = 0;
const noop = () => undefined;
assert.unaltered(noop, () => x);

Asynchronous asserts

Both the changer and changeWatcher callbacks can return a promise, or take a node-style callback, with error as the first parameter. If you provide a callback you need to give a final callback: option to the change assertion, that is used to notify your test runner that the test is complete.

With promises

Many test runners - for instance mocha - support simply returning promises from it() or test() blocks to support asynchronous tsts. chai-change supports this style.

If your runner doesn't support returning promises, you can use the .then() method to call a callback based API etc (or use callback: as in the error-first callback docs below.

 
it("creates a user", () => {
  let count = 0;
  const User = {
    create(attrs) {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          count += 1
          resolve();
        });
      });
    },
    count() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve(count);
        });
      });
    },
  };
 
  // when `changer` or `changeWatcher` returns a promise the expectation will return a promise as well
  return expect(() => (
    User.create({name: "bob"});
  )).to.alter(() => (
    User.count();
  ),{
    by: 1,
  });
})

With error-first callback

let count = 0;
const User = {
  create(attrs,cb) {
    setTimeout(() => {
      count += 1
      cb();
    });
  },
  count(cb) {
    setTimeout(() => {
      cb(null,count);
    });
  },
};
 
expect((stepDone) => {
  User.create({name: "bob"}, stepDone)
}).to.alter((stepDone) => {
  User.count(stepDone);
},{
  by: 1,
  callback: done
});

Tests

Node: npm install && npm test.

Browser: npm install then open test/index.html.

Changelog

2.1

Promise support - thanks to @talyssonoc!

Both the changeWatcher and changer functions can now return promises. The expectation also returns a promise when used with promises, which can be used directly with mocha etc.

2.0

• BREAKING CHANGE Change whole API from change to alter to avoid the .change method added to chai in chai@2.0.0.