diff --git a/app/webpack/javascripts/modules/determination.js b/app/webpack/javascripts/modules/determination.mjs
similarity index 100%
rename from app/webpack/javascripts/modules/determination.js
rename to app/webpack/javascripts/modules/determination.mjs
diff --git a/app/webpack/javascripts/modules/determination_spec.mjs b/app/webpack/javascripts/modules/determination_spec.mjs
new file mode 100644
index 0000000000..9df444dfb9
--- /dev/null
+++ b/app/webpack/javascripts/modules/determination_spec.mjs
@@ -0,0 +1,54 @@
+import { Determination } from './determination.mjs'
+
+describe('Determination', () => {
+  let determination = null
+
+  beforeEach(() => {
+    document.body.classList.add("govuk-frontend-supported")
+    document.body.append(`
+      <table id="determinations" data-module="govuk-determination" data-apply-vat="true" data-vat-url="/vat.json" data-submitted-date="2023-07-18" data-scheme="agfs">
+      </table>
+    `)
+
+    const elements = window.document.querySelectorAll('[data-module="govuk-determination"]')
+    determination = new Determination(elements[0])
+    determination.init()
+  })
+
+  describe('parsedValue', () => {
+    it('should parse a positive integer', () => {
+      const field = document.createElement('input')
+      field.value = '100'
+
+      expect(determination.parsedValue(field)).toEqual(100.0)
+    })
+
+    it('should parse a positive float', () => {
+      const field = document.createElement('input')
+      field.value = '99.99'
+
+      expect(determination.parsedValue(field)).toEqual(99.99)
+    })
+
+    it('should return zero for a negative integer', () => {
+      const field = document.createElement('input')
+      field.value = '-100'
+
+      expect(determination.parsedValue(field)).toEqual(0.0)
+    })
+
+    it('should return zero for a negative float', () => {
+      const field = document.createElement('input')
+      field.value = '-99.99'
+
+      expect(determination.parsedValue(field)).toEqual(0.0)
+    })
+
+    it('should return zero for a value that cannot be parsed', () => {
+      const field = document.createElement('input')
+      field.value = 'test'
+
+      expect(determination.parsedValue(field)).toEqual(0.0)
+    })
+  })
+})
diff --git a/app/webpack/packs/govuk-frontend.js b/app/webpack/packs/govuk-frontend.js
index 0ba7b9a2b5..31edeaf33e 100644
--- a/app/webpack/packs/govuk-frontend.js
+++ b/app/webpack/packs/govuk-frontend.js
@@ -1,6 +1,6 @@
 import '../stylesheets/govuk-frontend.scss'
 
-import { Determination } from '../javascripts/modules/determination.js'
+import { Determination } from '../javascripts/modules/determination.mjs'
 
 // fonts & images
 require.context('govuk-frontend/govuk/assets', true)
diff --git a/spec/support/jasmine-browser.json b/spec/support/jasmine-browser.json
index 827f170c8f..34f9b07446 100644
--- a/spec/support/jasmine-browser.json
+++ b/spec/support/jasmine-browser.json
@@ -3,12 +3,13 @@
   "srcFiles": [
     "app/assets/builds/*.js"
   ],
-  "specDir": "spec/javascripts",
+  "specDir": "./",
   "specFiles": [
-    "**/*[sS]pec.?(m)js"
+    "app/webpack/javascripts/modules/**/*[sS]pec.?(m)js",
+    "spec/javascripts/**/*[sS]pec.?(m)js"
   ],
   "helpers": [
-    "helpers/**/*.?(m)js"
+    "spec/javascripts/helpers/**/*.?(m)js"
   ],
   "env": {
     "stopSpecOnExpectationFailure": false,