fix: Use matching type for one when operating with valueless unit and a number (#3454)

Previously, when a valueless unit multiplied/divided something with a numerical value, it would just use ordinary 1 of type `number` in place of its null value. Now it matches the type of one used with the other numerical value.

Resolves #3450.
---------
Co-authored-by: Glen Whitney <[email protected]>

Author: josdejong

HISTORY.md

@@ -2,6 +2,8 @@
 
 # Unpublished changes since 14.4.0
 
+- Fix: #3450 support multiplication of valueless units by arbitrary types
+  (#3454).
 - Feat: increase performance of the `map` and `forEach` methods of 
   `DenseMatrix` (#3446). Thanks @dvd101x.
 

src/type/unit/Unit.js

@@ -657,8 +657,9 @@ export const createUnitClass = /* #__PURE__ */ factory(name, dependencies, ({
 
     // If at least one operand has a value, then the result should also have a value
     if (this.value !== null || other.value !== null) {
-      const valThis = this.value === null ? this._normalize(1) : this.value
-      const valOther = other.value === null ? other._normalize(1) : other.value
+      const valThis = this.value === null ? this._normalize(one(other.value)) : this.value
+      const valOther = other.value === null ? other._normalize(one(this.value)) : other.value
+
       res.value = multiplyScalar(valThis, valOther)
     } else {
       res.value = null
@@ -709,8 +710,8 @@ export const createUnitClass = /* #__PURE__ */ factory(name, dependencies, ({
 
     // If at least one operand has a value, the result should have a value
     if (this.value !== null || other.value !== null) {
-      const valThis = this.value === null ? this._normalize(1) : this.value
-      const valOther = other.value === null ? other._normalize(1) : other.value
+      const valThis = this.value === null ? this._normalize(one(other.value)) : this.value
+      const valOther = other.value === null ? other._normalize(one(this.value)) : other.value
       res.value = divideScalar(valThis, valOther)
     } else {
       res.value = null
@@ -772,6 +773,22 @@ export const createUnitClass = /* #__PURE__ */ factory(name, dependencies, ({
     }
   }
 
+  /**
+   * Create a value one with the numeric type of `typeOfValue`.
+   * For example, `one(new BigNumber(3))` returns `BigNumber(1)`
+   * @param {number | Fraction | BigNumber} typeOfValue
+   * @returns {number | Fraction | BigNumber}
+   */
+  function one (typeOfValue) {
+    // TODO: this is a workaround to prevent the following BigNumber conversion error from throwing:
+    //  "TypeError: Cannot implicitly convert a number with >15 significant digits to BigNumber"
+    //  see https://github.com/josdejong/mathjs/issues/3450
+    //      https://github.com/josdejong/mathjs/pull/3375
+    const convert = Unit._getNumberConverter(typeOf(typeOfValue))
+
+    return convert(1)
+  }
+
   /**
    * Calculate the absolute value of a unit
    * @memberof Unit

test/unit-tests/type/unit/Unit.test.js

@@ -1098,6 +1098,22 @@ describe('Unit', function () {
       assert.deepStrictEqual(unit4.divide(unit5), unit6)
     })
 
+    it('should multiply valueless units by any supported numeric type', function () {
+      const valuelessF = new Unit(null, 'fahrenheit')
+      assert.strictEqual(valuelessF.multiply(math.bignumber(123)).format(12), '123 fahrenheit')
+      assert.strictEqual(valuelessF.multiply(math.fraction(123, 2)).format(12), '123/2 fahrenheit')
+      assert.strictEqual(valuelessF.multiply(math.complex(123, 123)).format(12), '(123 + 123i) fahrenheit')
+      assert.strictEqual(valuelessF.multiply(123).format(12), '123 fahrenheit')
+    })
+
+    it('should divide valueless units by any supported numeric type', function () {
+      const valuelessF = new Unit(null, 'fahrenheit')
+      assert.strictEqual(valuelessF.divide(math.bignumber(1).div(123)).format(12), '123 fahrenheit')
+      assert.strictEqual(valuelessF.divide(math.fraction(2, 123)).format(12), '123/2 fahrenheit')
+      assert.strictEqual(valuelessF.divide(math.complex(0.25, 0.25)).format(12), '(2 - 2i) fahrenheit')
+      assert.strictEqual(valuelessF.divide(1 / 123).format(12), '123 fahrenheit')
+    })
+
     // eslint-disable-next-line mocha/no-skipped-tests
     it.skip('should cancel units in numerator and denominator', function () {
       assert.strictEqual(math.evaluate('2 J/K/g * 2 g').toString(), '4 J / K')