Assert This! is an experimental assertion library that leverages the proposed function-bind operator to create a clear left-to-right assertion style that doesn't require wrapping (like expect-style asssertions) or Object.prototype changes (like should-style assertions).

import {is, isTrue, isFalse} from 'assert-this/assertions';
import sum from './sum';
import isOdd from './is-odd';

sum(1, 2)::is(3);
isOdd(3)::isTrue();
isOdd(2)::isFalse();

They chain if you want to:

random(1, 5)::isAbove(1)::isBelow(5);

Since we're just dealing with functions, you can name them whatever you want:

import {
  is as betterBe,
  isTrue as betterBeTrue,
  isFalse as betterBeFalse,
} from 'assert-this/assertions';
import sum from './sum';
import isOdd from './is-odd';

sum(1, 2)::betterBe(3);
isOdd(3)::betterBeTrue();
isOdd(2)::betterBeFalse();

Also, since we're just dealing with functions, there's no need for plugins. To create your own assert-this-style assertions, just use assertThis.partial with a normal assert-style assertion function:

import assertThis from 'assert-this';
const betterBeGreaterThan5 = assertThis.partial((actual, msg) => {
  if (actual <= 5) throw new Error(msg || `${actual}  isn't greater than 5!`);
});

1::betterBeGreaterThan5();
2::betterBeGreaterThan5("This number's too small.");

Or just use Assert This! with regular assert-style functions at test time:

import assertThis from 'assert-this';
import {equal as equals} from 'assert'; // Plain old node assert module!
import sum from './sum';

sum(1, 2)::asssertThis(equals, 3);

In fact, this is really all the library does! For convenience, common assertions are included pre-wrapped in the 'assert-this/assertions' module (currently, they're wrapping chai's), but you don't have to use them if you don't want to.

I'm not a big fan of expect-style assertions. The expect() function returns a wrapper object with assertion methods. If you want to make custom assertions (and maintain the same style), you'll need to add them to the wrapper object. This means plugins, a shared namespace, and possible namespace collisions. Regular functions, on the other hand, don't have these issues.

At the same time, I get why people like the expect() style. It reads well from left-to-right and there's no confusion about which is the expected value and which is the actual. Needing to use a wrapper object to accomplish this is a shortcoming in the language.

This project is an attempt to get the best of both worlds.

This is an experiment. Stuff might change.