examples of let like behavior in jest

incomplete.js

// For a more complete version of this look at given: https://github.com/tatyshev/given2/
// it supports more options and has more checks, but is also much bigger than this
// 15 line approach.

const c = {};

// Approach #1 add properties to a variable `c`
// these use a getter that gets replaced after being called the first time
// In theory this can be used with the global object, but that means your test would be adding
// properties to the global namespace
// This approach hasn't been tested well though. I kind of think it doesn't work as expected since the
// beforeEach here might not know its context. I'm not sure how Jest keeps track of the context. It does seem to work
// though. So perhaps Jest builds up a tree of blocks and has an internal context that the beforeEach call addes to.
function set(name, computeValue) {
  beforeEach(() => {
    Object.defineProperty(c, name, {
      configurable: true,
      get: () => {
        delete c[name];
        return c[name] = computeValue();
      }
    });
  });
  afterEach(() => {
    delete c[name];
  });
}

// Approach #2, use a simple memoization function
// this would be used like `let variable = mem(() => value);`
// In this case overriding a variable later requires putting it in a beforeEach block
// it seems Jest evaluates all of the describe blocks before running the first it block
// this makes sense. But it means any overrides will be called and then modify the variable
// before it is actually used.
// because of this the set approach above seems best
function mem(computeValue) {
  let computed = false;
  let value;
  return () => {
    if(computed) { 
      return value 
    } else {
      computed = true;
      return value = computeValue();
    }
  }
}

describe("<NumericTextField /> minValue prop", () => {
  let props = mem(() => ({}));
  let wrapper = mem(() => shallow(<NumericTextField {...props()}/>));
  
  // set("wrapper", () => shallow(<NumericTextField {...c.props}/>) );
  // set("props", () => ({}) );

  describe("default behavior with no configuration", () => {
    it("should not allow non-numeric values, and default to 0", () => {
      wrapper().find("input").simulate("change", { target: { value: "x" } });
      wrapper().find("input").simulate("blur", {});
      expect(wrapper().state("value")).toBe(0);
    });
    it("will allow values greater than 0", () => {
      c.wrapper.find("input").simulate("change", { target: { value: "5" } });
      c.wrapper.find("input").simulate("blur", {});
      expect(c.wrapper.state("value")).toBe(5);
    });
    it("will allow an in-range decimal value. Truncates to integer", () => {
      c.wrapper.find("input").simulate("change", { target: { value: "2.6" } });
      c.wrapper.find("input").simulate("blur", {});
      expect(c.wrapper.state("value")).toBe(2);
    });
  });