A very basic implementation of `expect` as seen in mocha/jasmine test libraries to help new coders understand some of the magic behind testing and method/property chaining.

expect_assertion_inner_workings_example.js

// A super simple no-edge case version of the `expect` function 
// as seen in mocha/jasmine test environments.

// Long form

// step one: define a function called `expect` which takes one value. 
// This value is intended to be the return value of a function.
const expect = function(actualValue) {
  // assign a property onto `expect` called 'equalsTo' which is 
  // itself  a function taking one argument (using `this` keyword 
  // which references the object itself in this case the function 
  // `expect`, since its uses the `function` keyword in its declaration).
  // Properties can be assigned onto fns since functions are just a
  // slightly special object (object is a collection of {key: value} pairs).
  this.equalsTo = function(expectedValue) {
    // `equalsTo` returns true or false depending on if `===` comparsion
    // is successful.
    return actualValue === expectedValue
  }
  // return the `expect` function with the `equalsTo` property which we can now call.
  // It basically returns a "tranformed" version of itself which can dynamically change
  // depending on the input of `actualValue`.
  return this
}

// Arrow function internal
const expect = function(actualValue) {
  this.equalsTo = (expectedValue) => { 
    return actualValue === expectedValue
  }
  return this
}

// 1-line auto-return arrow function
const expect3 = function(actualValue) {
  this.equalsTo = expectedValue => actualValue === expectedValue
  return this
}

// No THIS version.
// Return a plain object instead with an `equalsTo` key which corresponds 
// to a short form anonymous function. `expect4` is basically a function 
// which returns a partially filled function using its own argument `actualValue`.
const expect4 = function(actualValue) {
   return { 
     equalsTo: expectedValue => expectedValue === actualValue
   }
}


// To use, all the same.
expect('bob').toEqual('bob'); // => true
expect('bob').toEqual('tom'); // => false

expect2('bob').toEqual('bob'); // => true
expect2('bob').toEqual('tom'); // => false

expect3('bob').toEqual('bob'); // => true
expect3('bob').toEqual('tom'); // => false

expect4('bob').toEqual('bob'); // => true
expect4('bob').toEqual('tom'); // => false