JavaScript testing with lazy loading/evaluation (like Rspec let)

js-test-lazy-evaluate.js

// Uses Jasmine's `this` feature:
// https://jasmine.github.io/2.9/introduction#section-The_<code>this</code>_keyword


// Example without lazy evaluation:

const sum = (a, b) => a + b;

describe('No lazy eval :(', () => {
  let a, b, total;

  beforeEach(() => {
    a = 5;
    b = 3;
    total = sum(a, b); // bleh
  });

  it('sums', () => {
    expect(total).toEqual(8);
  });

  it('sums with b changed', () => {
    b = 5;
    total = sum(a, b); // boo
    expect(total).toEqual(10);
  });

  it('sums with both changed', () => {
    a = 10;
    b = 8;
    total = sum(a, b); // hiss
    expect(total).toEqual(18);
  });
});


// WITH lazy evaluation:

// This can go in a test_helper file and be imported.
// Note: beforeEach() must use function() {} and not () => {}
beforeEach(function() {
  this.$lazy = (propName, getter) => {
    let lazy;
    Object.defineProperty(this, propName, {
      get() {
        lazy = lazy || getter.call(this);
        return lazy;
      },
      set() {},
      enumerable: true,
      configurable: true,
    });
  };
});

const sum = (a, b) => a + b;

// beforeEach() and it() must use function() {} syntax
describe('Lazy eval! :D', () => {
  beforeEach(function() {
    this.a = 5;
    this.b = 3;
    this.$lazy('total', () => sum(this.a, this.b));
  });

  it('sums', function() {
    expect(this.total).toEqual(8);
  });

  it('sums with b changed', function() {
    this.b = 5;
    expect(this.total).toEqual(10);
  });

  it('sums with both changed', function() {
    this.a = 10;
    this.b = 8;
    expect(this.total).toEqual(18);
  });
});