Proof for function composition

compose.test.js

import assert from "assert";

const compose = (...functions) => (initialArg) =>
  functions.reduceRight(
    (accumulatedValue, func) => func(accumulatedValue),
    initialArg
  );

describe("compose", () => {
  it("returns a composed function h, so that h(x) is equal to f(g(x))", () => {
    const x = 300;

    const f = (x) => x + 1000;
    const g = (x) => x + 37;
    const h = compose(f, g);

    const outputfromG = g(x);
    const outputFromF = f(outputfromG);    
    const outputFromH = h(x);
    
    const expectedOutput = 1337;
    
    assert.equal(outputFromF, expectedOutput);
    assert.equal(outputFromH, expectedOutput);
    assert.equal(outputFromH, outputFromF);
  });
});