test: Set up a mocha test harness (#5)

We use [mocha](https://mochajs.org/) as the test framework, as it is
  the tool used by mathjs and we would like to make tests as similar
  as possible. However, to tighten the linkage between source code and
  tests, we adopt a somewhat different file organization: unit tests
  for a given source file `blah/foo.js` are in `blah/__test__/foo.spec.js`.

  To run all unit tests, execute the script `pnpm test`.

  Resolves #3.

Reviewed-on: glen/nanomath#5
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>

src/number/Number.js

@@ -1,3 +1,3 @@
-import {Type} from '../core/Type.js'
+import {Type} from '#core/Type.js'
 
 export const Number = new Type(n => typeof n === 'number')

src/number/__test__/arithmetic.spec.js

@@ -0,0 +1,17 @@
+import assert from 'assert'
+import math from '#nanomath'
+
+describe('number arithmetic', () => {
+   it('supports basic operations', () => {
+      assert.strictEqual(math.abs(-Infinity), Infinity)
+      assert.strictEqual(math.absquare(-2), 4)
+      assert.strictEqual(math.add(2, 2), 4)
+      assert.strictEqual(math.divide(6, 4), 1.5)
+      assert.strictEqual(math.cbrt(-8), -2)
+      assert.strictEqual(math.invert(-4), -0.25)
+      assert.strictEqual(math.multiply(4, 1.5), 6)
+      assert.strictEqual(math.negate(Infinity), -Infinity)
+      assert.strictEqual(math.subtract(4, 2), 2)
+      assert.strictEqual(math.quotient(7, 3), 2)
+   })
+})

src/number/arithmetic.js

@@ -10,7 +10,7 @@ export const cbrt = plain(a => {
    if (negate) a = -a
    let result = a
    if (isFinite(a)) {
-      result = Math.exp(Math.log(x) / 3)
+      result = Math.exp(Math.log(result) / 3)
       result = (a / (result * result) + (2 * result)) / 3
    }
    return negate ? -result : result
@@ -18,4 +18,5 @@ export const cbrt = plain(a => {
 export const invert = plain(a => 1/a)
 export const multiply = plain((a, b) => a * b)
 export const negate = plain(a => -a)
+export const subtract = plain((a, b) => a - b)
 export const quotient = plain((a,b) => Math.floor(a/b))

src/number/helpers.js

@@ -1,4 +1,4 @@
-import {Returns, onType} from '../core/helpers.js'
+import {Returns, onType} from '#core/helpers.js'
 import {Number} from './Number.js'
 
 export const plain = f => onType(