Test more elements of the fn file

fn.js

@@ -16,7 +16,7 @@ function arg2(_arg1, arg2){
 function filterEnvelope(low,high, element){
 	return function(o){
 		const value = element(o);
-		return low < value &&  value <= high;
+		return low <= value &&  value <= high;
 	}
 }
 

fn.test.js

@@ -0,0 +1,72 @@
+const {expect} = require("chai");
+const {identity, arg2} = require("./fn");
+
+describe("Identity", function(){
+	describe("when invoked", function(){
+		it("gives the same value", function () {
+			expect(identity(1)).to.eq(1);
+		});
+	})
+});
+
+describe("arg2", function () {
+	describe("when given two values", function () {
+		it("returns the second value", function () {
+			expect(arg2(1,2)).to.eq(2);
+		});
+	});
+});
+
+const {filterEnvelope} = require("./fn");
+describe("filterEnvelope", function (){
+	describe("When given a low value", function () {
+		it("is falsy", function () {
+			expect(filterEnvelope(10,20, identity)(1)).to.be.false;
+		});
+	});
+
+	describe("When given a high value", function () {
+		it("is falsy", function () {
+			expect(filterEnvelope(10,20, identity)(21)).to.be.false;
+		});
+	});
+
+	describe("When given a value within the envelope", function () {
+		it("is truthy", function () {
+			expect(filterEnvelope(10,20, identity)(15)).to.be.true;
+		});
+	});
+
+	describe("When given a value on the low side of the envelope", function () {
+		it("is truthy", function () {
+			expect(filterEnvelope(10,20, identity)(10)).to.be.true;
+		});
+	});
+
+	describe("When given a value on the high side of the envelope", function () {
+		it("is truthy", function () {
+			expect(filterEnvelope(10,20, identity)(20)).to.be.true;
+		});
+	});
+});
+
+const {y} = require("./fn");
+describe("y combinator", function () {
+	describe("when given no functions", function () {
+		it("returns the input", function () {
+			expect(y()(42)).to.eq(42);
+		})
+	});
+
+	describe("when given a set of functions", function () {
+		it("calls each one", function () {
+			let called = 0;
+			const call = (c) => {
+				called++;
+				return called + c;
+			};
+			expect(y(call,call,call)(1)).to.eq(7);
+			expect(called).to.eq(3);
+		})
+	});
+});