diff --git a/src/background/repositories/PersistentSettingRepository.ts b/src/background/repositories/PersistentSettingRepository.ts
index 927bce9..e3b78b3 100644
--- a/src/background/repositories/PersistentSettingRepository.ts
+++ b/src/background/repositories/PersistentSettingRepository.ts
@@ -8,7 +8,7 @@ export default class SettingRepository {
     if (!settings) {
       return null;
     }
-    return SettingData.valueOf(settings as any);
+    return SettingData.fromJSON(settings as any);
   }
 }
 
diff --git a/src/settings/components/form/KeymapsForm.tsx b/src/settings/components/form/KeymapsForm.tsx
index 3cba0c0..94934ae 100644
--- a/src/settings/components/form/KeymapsForm.tsx
+++ b/src/settings/components/form/KeymapsForm.tsx
@@ -12,7 +12,7 @@ interface Props {
 
 class KeymapsForm extends React.Component<Props> {
   public static defaultProps: Props = {
-    value: FormKeymaps.valueOf({}),
+    value: FormKeymaps.fromJSON({}),
     onChange: () => {},
     onBlur: () => {},
   };
diff --git a/src/settings/components/form/SearchForm.tsx b/src/settings/components/form/SearchForm.tsx
index 6ba6cfb..0aaf6fd 100644
--- a/src/settings/components/form/SearchForm.tsx
+++ b/src/settings/components/form/SearchForm.tsx
@@ -12,7 +12,7 @@ interface Props {
 
 class SearchForm extends React.Component<Props> {
   public static defaultProps: Props = {
-    value: FormSearch.valueOf({ default: '', engines: []}),
+    value: FormSearch.fromJSON({ default: '', engines: []}),
     onChange: () => {},
     onBlur: () => {},
   };
@@ -81,7 +81,7 @@ class SearchForm extends React.Component<Props> {
       }
     }
 
-    this.props.onChange(FormSearch.valueOf(next));
+    this.props.onChange(FormSearch.fromJSON(next));
     if (name === 'delete' || name === 'default') {
       this.props.onBlur();
     }
diff --git a/src/settings/components/index.tsx b/src/settings/components/index.tsx
index 586f0f0..160dd9c 100644
--- a/src/settings/components/index.tsx
+++ b/src/settings/components/index.tsx
@@ -133,7 +133,7 @@ class SettingsComponent extends React.Component<Props> {
     let data = new SettingData({
       source: this.props.source,
       form: (this.props.form as FormSettings).buildWithSearch(
-        FormSearch.valueOf(value)),
+        FormSearch.fromJSON(value)),
     });
     this.props.dispatch(settingActions.set(data));
   }
diff --git a/src/settings/storage.ts b/src/settings/storage.ts
index 32b6351..2a983df 100644
--- a/src/settings/storage.ts
+++ b/src/settings/storage.ts
@@ -6,7 +6,7 @@ export const load = async(): Promise<SettingData> => {
     return DefaultSettingData;
   }
   try {
-    return SettingData.valueOf(settings as any);
+    return SettingData.fromJSON(settings as any);
   } catch (e) {
     console.error('unable to load settings', e);
     return DefaultSettingData;
diff --git a/src/shared/SettingData.ts b/src/shared/SettingData.ts
index b19a756..532570e 100644
--- a/src/shared/SettingData.ts
+++ b/src/shared/SettingData.ts
@@ -6,9 +6,9 @@ import Properties from './settings/Properties';
 import Blacklist from './settings/Blacklist';
 
 export class FormKeymaps {
-  private data: {[op: string]: string};
+  private readonly data: {[op: string]: string};
 
-  constructor(data: {[op: string]: string}) {
+  private constructor(data: {[op: string]: string}) {
     this.data = data;
   }
 
@@ -38,7 +38,7 @@ export class FormKeymaps {
     return new FormKeymaps(newData);
   }
 
-  static valueOf(o: ReturnType<FormKeymaps['toJSON']>): FormKeymaps {
+  static fromJSON(o: ReturnType<FormKeymaps['toJSON']>): FormKeymaps {
     let data: {[op: string]: string} = {};
     for (let op of Object.keys(o)) {
       data[op] = o[op] as string;
@@ -65,9 +65,9 @@ export class FormKeymaps {
 }
 
 export class FormSearch {
-  private default: string;
+  private readonly default: string;
 
-  private engines: string[][];
+  private readonly engines: string[][];
 
   constructor(defaultEngine: string, engines: string[][]) {
     this.default = defaultEngine;
@@ -92,7 +92,7 @@ export class FormSearch {
     };
   }
 
-  static valueOf(o: ReturnType<FormSearch['toJSON']>): FormSearch {
+  static fromJSON(o: ReturnType<FormSearch['toJSON']>): FormSearch {
     if (!Object.prototype.hasOwnProperty.call(o, 'default')) {
       throw new TypeError(`"default" field not set`);
     }
@@ -220,15 +220,15 @@ export class FormSettings {
     };
   }
 
-  static valueOf(o: ReturnType<FormSettings['toJSON']>): FormSettings {
+  static fromJSON(o: ReturnType<FormSettings['toJSON']>): FormSettings {
     for (let name of ['keymaps', 'search', 'properties', 'blacklist']) {
       if (!Object.prototype.hasOwnProperty.call(o, name)) {
         throw new Error(`"${name}" field not set`);
       }
     }
     return new FormSettings(
-      FormKeymaps.valueOf(o.keymaps),
-      FormSearch.valueOf(o.search),
+      FormKeymaps.fromJSON(o.keymaps),
+      FormSearch.fromJSON(o.search),
       Properties.fromJSON(o.properties),
       Blacklist.fromJSON(o.blacklist),
     );
@@ -311,7 +311,7 @@ export default class SettingData {
     throw new Error(`unknown settings source: ${this.source}`);
   }
 
-  static valueOf(o: {
+  static fromJSON(o: {
     source: string;
     json?: string;
     form?: ReturnType<FormSettings['toJSON']>;
@@ -326,7 +326,7 @@ export default class SettingData {
     case SettingSource.Form:
       return new SettingData({
         source: o.source,
-        form: FormSettings.valueOf(
+        form: FormSettings.fromJSON(
           o.form as ReturnType<FormSettings['toJSON']>),
       });
     }
@@ -334,7 +334,7 @@ export default class SettingData {
   }
 }
 
-export const DefaultSettingData: SettingData = SettingData.valueOf({
+export const DefaultSettingData: SettingData = SettingData.fromJSON({
   source: 'json',
   json: DefaultSettingJSONText,
 });
diff --git a/test/settings/components/form/KeymapsForm.test.tsx b/test/settings/components/form/KeymapsForm.test.tsx
index dc2322b..1d1e77c 100644
--- a/test/settings/components/form/KeymapsForm.test.tsx
+++ b/test/settings/components/form/KeymapsForm.test.tsx
@@ -9,7 +9,7 @@ import { expect } from 'chai';
 describe("settings/form/KeymapsForm", () => {
   describe('render', () => {
     it('renders keymap fields', () => {
-      let root = ReactTestRenderer.create(<KeymapsForm value={FormKeymaps.valueOf({
+      let root = ReactTestRenderer.create(<KeymapsForm value={FormKeymaps.fromJSON({
         'scroll.vertically?{"count":1}': 'j',
         'scroll.vertically?{"count":-1}': 'k',
       })} />).root
@@ -48,7 +48,7 @@ describe("settings/form/KeymapsForm", () => {
     it('invokes onChange event on edit', (done) => {
       ReactTestUtils.act(() => {
         ReactDOM.render(<KeymapsForm
-          value={FormKeymaps.valueOf({
+          value={FormKeymaps.fromJSON({
             'scroll.vertically?{"count":1}': 'j',
             'scroll.vertically?{"count":-1}': 'k',
           })}
diff --git a/test/settings/components/form/SearchEngineForm.test.tsx b/test/settings/components/form/SearchEngineForm.test.tsx
index 0e6b17d..1f0420d 100644
--- a/test/settings/components/form/SearchEngineForm.test.tsx
+++ b/test/settings/components/form/SearchEngineForm.test.tsx
@@ -8,7 +8,7 @@ import { FormSearch } from 'shared/SettingData';
 describe("settings/form/SearchForm", () => {
   describe('render', () => {
     it('renders SearchForm', () => {
-      let root = ReactTestRenderer.create(<SearchForm value={FormSearch.valueOf({
+      let root = ReactTestRenderer.create(<SearchForm value={FormSearch.fromJSON({
         default: 'google',
         engines: [['google', 'google.com'], ['yahoo', 'yahoo.com']],
       })} />).root;
@@ -41,7 +41,7 @@ describe("settings/form/SearchForm", () => {
     it('invokes onChange event on edit', (done) => {
       ReactTestUtils.act(() => {
         ReactDOM.render(<SearchForm
-          value={FormSearch.valueOf({
+          value={FormSearch.fromJSON({
             default: 'google',
               engines: [['google', 'google.com'], ['yahoo', 'yahoo.com']]
           })}
@@ -67,7 +67,7 @@ describe("settings/form/SearchForm", () => {
 
     it('invokes onChange event on delete', (done) => {
       ReactTestUtils.act(() => {
-        ReactDOM.render(<SearchForm value={FormSearch.valueOf({
+        ReactDOM.render(<SearchForm value={FormSearch.fromJSON({
             default: 'yahoo',
             engines: [['louvre', 'google.com'], ['yahoo', 'yahoo.com']]
           })}
@@ -88,7 +88,7 @@ describe("settings/form/SearchForm", () => {
 
     it('invokes onChange event on add', (done) => {
       ReactTestUtils.act(() => {
-        ReactDOM.render(<SearchForm value={FormSearch.valueOf({
+        ReactDOM.render(<SearchForm value={FormSearch.fromJSON({
           default: 'yahoo',
             engines: [['google', 'google.com']]
           })}
diff --git a/test/shared/SettingData.test.ts b/test/shared/SettingData.test.ts
index 138d768..d95c271 100644
--- a/test/shared/SettingData.test.ts
+++ b/test/shared/SettingData.test.ts
@@ -14,7 +14,7 @@ describe('shared/SettingData', () => {
           'scroll.home': '0',
         };
 
-        let keymaps = FormKeymaps.valueOf(data).toKeymaps().toJSON();
+        let keymaps = FormKeymaps.fromJSON(data).toKeymaps().toJSON();
         expect(keymaps).to.deep.equal({
           'j': { type: 'scroll.vertically', count: 1 },
           '0': { type: 'scroll.home' },
@@ -118,7 +118,7 @@ describe('shared/SettingData', () => {
           blacklist: []
         };
 
-        let settings = FormSettings.valueOf(data).toSettings();
+        let settings = FormSettings.fromJSON(data).toSettings();
         expect({
           keymaps: settings.keymaps.toJSON(),
           search: settings.search.toJSON(),
@@ -211,7 +211,7 @@ describe('shared/SettingData', () => {
           }`,
         };
 
-        let j = SettingData.valueOf(data).toJSON();
+        let j = SettingData.fromJSON(data).toJSON();
         expect(j.source).to.equal('json');
         expect(j.json).to.be.a('string');
       });
@@ -236,7 +236,7 @@ describe('shared/SettingData', () => {
           },
         };
 
-        let j = SettingData.valueOf(data).toJSON();
+        let j = SettingData.fromJSON(data).toJSON();
         expect(j.source).to.equal('form');
         expect(j.form).to.deep.equal({
           keymaps: {},
@@ -277,7 +277,7 @@ describe('shared/SettingData', () => {
           }`,
         };
 
-        let settings = SettingData.valueOf(data).toSettings();
+        let settings = SettingData.fromJSON(data).toSettings();
         expect(settings.search.defaultEngine).to.equal('google');
       });
 
@@ -301,7 +301,7 @@ describe('shared/SettingData', () => {
           },
         };
 
-        let settings = SettingData.valueOf(data).toSettings();
+        let settings = SettingData.fromJSON(data).toSettings();
         expect(settings.search.defaultEngine).to.equal('yahoo');
       });
     });