add ability to do arithmetics with unit classes

src/units/force/force.ts

@@ -1,42 +1,41 @@
+import Unit from '../unit';
 import type { ForceUnit } from './types';
 
-class Force {
-  private stdUnit: number;
-
+class Force extends Unit {
   constructor(value: number, unit: ForceUnit) {
-    this.stdUnit = Force.#parseToStd(value, unit);
+    super(Force.#parseToStd(value, unit));
   }
 
   N() {
-    return this.stdUnit;
+    return this.stdValue;
   }
 
   kN() {
-    return this.stdUnit / 1000;
+    return this.stdValue / 1000;
   }
 
   lbf() {
-    return this.stdUnit / 4.448;
+    return this.stdValue / 4.448;
   }
 
   kip() {
-    return this.stdUnit / 4448.222;
+    return this.stdValue / 4448.222;
   }
 
   kgf() {
-    return this.stdUnit / 9.807;
+    return this.stdValue / 9.807;
   }
 
   MN() {
-    return this.stdUnit / 1000000;
+    return this.stdValue / 1000000;
   }
 
   tonf() {
-    return this.stdUnit / 9807;
+    return this.stdValue / 9807;
   }
 
   set(value: number, unit: ForceUnit) {
-    this.stdUnit = Force.#parseToStd(value, unit);
+    this.stdValue = Force.#parseToStd(value, unit);
   }
 
   static #parseToStd(value: number, unit: ForceUnit) {

src/units/length/length.ts

@@ -1,10 +1,9 @@
+import Unit from '../unit';
 import type { LengthUnit } from './types';
 
-class Length {
-  private stdValue: number;
-
+class Length extends Unit {
   constructor(value: number, unit: LengthUnit) {
-    this.stdValue = Length.#parseToStd(value, unit);
+    super(Length.#parseToStd(value, unit));
   }
 
   mm() {

src/units/stress/stress.ts

@@ -1,10 +1,9 @@
+import Unit from '../unit';
 import type { StressUnit } from './types';
 
-class Stress {
-  private stdValue: number;
-
+class Stress extends Unit {
   constructor(value: number, unit: StressUnit) {
-    this.stdValue = Stress.#parseToStd(value, unit);
+    super(Stress.#parseToStd(value, unit));
   }
 
   MPa() {

src/units/time/time.ts

@@ -1,10 +1,9 @@
+import Unit from '../unit';
 import type { TimeUnit } from './types';
 
-class Time {
-  private stdValue: number;
-
+class Time extends Unit {
   constructor(value: number, unit: TimeUnit) {
-    this.stdValue = Time.#parseToStd(value, unit);
+    super(Time.#parseToStd(value, unit));
   }
 
   ms() {

src/units/unit/index.ts

@@ -0,0 +1 @@
+export { default } from './unit';

src/units/unit/unit.test.ts

@@ -0,0 +1,15 @@
+import Unit from '.';
+
+describe('Unit', () => {
+  const something = new Unit(100);
+
+  describe('valueOf', () => {
+    it('returns the stdValue', () => {
+      expect(something.valueOf()).toBe(100);
+    });
+
+    it('enables arithmetic operations', () => {
+      expect(+something + 100).toBe(200);
+    });
+  });
+});

src/units/unit/unit.ts

@@ -0,0 +1,13 @@
+class Unit {
+  stdValue: number;
+
+  constructor(value: number) {
+    this.stdValue = value;
+  }
+
+  valueOf() {
+    return this.stdValue;
+  }
+}
+
+export default Unit;