Update development dependencies

Closes #14
Closes #15

package.json

@@ -26,22 +26,22 @@
     "parser": "@typescript-eslint/parser",
     "extends": [
       "plugin:github/browser",
-      "plugin:github/es6",
+      "plugin:github/recommended",
       "plugin:github/typescript"
     ]
   },
   "devDependencies": {
     "@github/prettier-config": "0.0.4",
     "chai": "^4.2.0",
-    "eslint": "^6.8.0",
-    "eslint-plugin-github": "^3.4.1",
-    "karma": "^5.0.1",
+    "eslint": "^7.9.0",
+    "eslint-plugin-github": "^4.1.1",
+    "karma": "^5.2.2",
     "karma-chai": "^0.1.0",
     "karma-chrome-launcher": "^3.1.0",
-    "karma-mocha": "^1.3.0",
+    "karma-mocha": "^2.0.1",
     "karma-mocha-reporter": "^2.2.5",
-    "mocha": "^7.1.1",
-    "typescript": "^3.8.3"
+    "mocha": "^8.1.3",
+    "typescript": "^4.0.2"
   },
   "dependencies": {},
   "eslintIgnore": [

src/index.ts

@@ -1,14 +1,14 @@
 // Last submitted HTMLFormElement that will cause a browser navigation.
 let submittedForm: HTMLFormElement | null = null
 
-function shouldResumeField(field: HTMLInputElement | HTMLTextAreaElement) {
-  return field.id && field.value !== field.defaultValue && field.form !== submittedForm
+function shouldResumeField(field: HTMLInputElement | HTMLTextAreaElement): boolean {
+  return !!field.id && field.value !== field.defaultValue && field.form !== submittedForm
 }
 
 type PersistOptions = {selector?: string; keyPrefix?: string}
 
 // Write all ids and values of the selected fields on the page into sessionStorage.
-export function persistResumableFields(id: string, options?: PersistOptions) {
+export function persistResumableFields(id: string, options?: PersistOptions): void {
   const selector = options?.selector ?? '.js-session-resumable'
   const keyPrefix = options?.keyPrefix ?? 'session-resume:'
   const key = `${keyPrefix}${id}`
@@ -33,7 +33,7 @@ export function persistResumableFields(id: string, options?: PersistOptions) {
 
 type RestoreOptions = {keyPrefix?: string}
 
-export function restoreResumableFields(id: string, options?: RestoreOptions) {
+export function restoreResumableFields(id: string, options?: RestoreOptions): void {
   const keyPrefix = options?.keyPrefix ?? 'session-resume:'
   const key = `${keyPrefix}${id}`
   let fields
@@ -81,7 +81,7 @@ export function restoreResumableFields(id: string, options?: RestoreOptions) {
   }, 0)
 }
 
-export function setForm(event: Event) {
+export function setForm(event: Event): void {
   submittedForm = event.target as HTMLFormElement
 
   setTimeout(function () {