diff --git a/extension/background/checkable.ts b/extension/background/checkable.ts
new file mode 100644
index 000000000..63fbd3646
--- /dev/null
+++ b/extension/background/checkable.ts
@@ -0,0 +1,87 @@
+/*
+ This file is part of TALER
+ (C) 2016 GNUnet e.V.
+
+ TALER is free software; you can redistribute it and/or modify it under the
+ terms of the GNU General Public License as published by the Free Software
+ Foundation; either version 3, or (at your option) any later version.
+
+ TALER is distributed in the hope that it will be useful, but WITHOUT ANY
+ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License along with
+ TALER; see the file COPYING.  If not, If not, see <http://www.gnu.org/licenses/>
+ */
+
+"use strict";
+
+/**
+ * Decorators for type-checking JSON into
+ * an object.
+ * @module Checkable
+ * @author Florian Dold
+ */
+
+
+namespace Checkable {
+  let chkSym = Symbol("checkable");
+
+  function checkNumber(target, prop) {
+    return true;
+  }
+
+  function checkString(target, prop) {
+    return true;
+  }
+
+  export function Class(target) {
+    target.checked = (v) => {
+      let props = target.prototype[chkSym].props;
+      console.log("hello, world");
+      let remainingPropNames = new Set(Object.getOwnPropertyNames(v));
+
+      for (let prop of props) {
+        remainingPropNames.delete(prop);
+        console.log("prop", prop);
+      }
+
+      if (remainingPropNames.size != 0) {
+        throw Error("superfluous properties " + JSON.stringify(remainingPropNames.values()));
+      }
+    };
+    return target;
+  }
+
+  export function Value(type) {
+    function deco(target) {
+    }
+
+    return deco;
+  }
+
+  export function List(type) {
+    function deco(target) {
+    }
+
+    return deco;
+  }
+
+  export function Number(target: Object, propertyKey: string | symbol): void {
+    let chk = target[chkSym];
+    if (!chk) {
+      chk = {props: []};
+      target[chkSym] = chk;
+    }
+    chk.props.push({propertyKey: propertyKey, checker: checkNumber});
+  }
+
+  export function String(target: Object, propertyKey: string | symbol): void {
+    let chk = target[chkSym];
+    if (!chk) {
+      chk = {props: []};
+      target[chkSym] = chk;
+    }
+    chk.props.push({propertyKey: propertyKey, checker: checkString});
+  }
+}
diff --git a/extension/background/db.js b/extension/background/db.js
index 0e4576851..4b81555da 100644
--- a/extension/background/db.js
+++ b/extension/background/db.js
@@ -55,7 +55,7 @@ function exportDb(db) {
     };
     return new Promise((resolve, reject) => {
         let tx = db.transaction(db.objectStoreNames);
-        tx.addEventListener('complete', (e) => {
+        tx.addEventListener("complete", (e) => {
             resolve(dump);
         });
         for (let i = 0; i < db.objectStoreNames.length; i++) {
@@ -64,7 +64,7 @@ function exportDb(db) {
             dump.stores[name] = storeDump;
             let store = tx.objectStore(name)
                 .openCursor()
-                .addEventListener('success', (e) => {
+                .addEventListener("success", (e) => {
                 let cursor = e.target.result;
                 if (cursor) {
                     storeDump[cursor.key] = cursor.value;
diff --git a/extension/background/db.ts b/extension/background/db.ts
index df24e1471..d3c6e9182 100644
--- a/extension/background/db.ts
+++ b/extension/background/db.ts
@@ -41,10 +41,10 @@ namespace Db {
   }
 
   export interface Denomination {
-    value: AmountJson;
+    value: AmountJson_interface;
     denom_pub: string;
-    fee_withdraw: AmountJson;
-    fee_deposit: AmountJson;
+    fee_withdraw: AmountJson_interface;
+    fee_deposit: AmountJson_interface;
   }
 
   export interface PreCoin {
@@ -56,7 +56,7 @@ namespace Db {
     withdrawSig: string;
     coinEv: string;
     mintBaseUrl: string;
-    coinValue: AmountJson;
+    coinValue: AmountJson_interface;
   }
 
   export interface Coin {
@@ -64,7 +64,7 @@ namespace Db {
     coinPriv: string;
     denomPub: string;
     denomSig: string;
-    currentAmount: AmountJson;
+    currentAmount: AmountJson_interface;
     mintBaseUrl: string;
   }
 
@@ -119,7 +119,7 @@ function exportDb(db): Promise<any> {
   return new Promise((resolve, reject) => {
 
     let tx = db.transaction(db.objectStoreNames);
-    tx.addEventListener('complete', (e) => {
+    tx.addEventListener("complete", (e) => {
       resolve(dump);
     });
     for (let i = 0; i < db.objectStoreNames.length; i++) {
@@ -128,7 +128,7 @@ function exportDb(db): Promise<any> {
       dump.stores[name] = storeDump;
       let store = tx.objectStore(name)
                     .openCursor()
-                    .addEventListener('success', (e) => {
+                    .addEventListener("success", (e) => {
                       let cursor = e.target.result;
                       if (cursor) {
                         storeDump[cursor.key] = cursor.value;
diff --git a/extension/background/emscriptif.ts b/extension/background/emscriptif.ts
index 6b9d5014a..691c27379 100644
--- a/extension/background/emscriptif.ts
+++ b/extension/background/emscriptif.ts
@@ -291,7 +291,7 @@ arenaStack.push(new SyncArena());
 
 
 class Amount extends ArenaObject {
-  constructor(args?: AmountJson, arena?: Arena) {
+  constructor(args?: AmountJson_interface, arena?: Arena) {
     super(arena);
     if (args) {
       this.nativePtr = emscAlloc.get_amount(args.value,
diff --git a/extension/background/wallet.js b/extension/background/wallet.js
index ebb04e832..120d177eb 100644
--- a/extension/background/wallet.js
+++ b/extension/background/wallet.js
@@ -21,7 +21,29 @@
  */
 /// <reference path="../decl/urijs/URIjs.d.ts" />
 /// <reference path="../decl/chrome/chrome.d.ts" />
-'use strict';
+"use strict";
+var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
+    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
+    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
+    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
+    return c > 3 && r && Object.defineProperty(target, key, r), r;
+};
+class MyClass {
+}
+let AmountJson = class {
+};
+__decorate([
+    Checkable.Number
+], AmountJson.prototype, "value", void 0);
+__decorate([
+    Checkable.Number
+], AmountJson.prototype, "fraction", void 0);
+__decorate([
+    Checkable.String
+], AmountJson.prototype, "currency", void 0);
+AmountJson = __decorate([
+    Checkable.Class
+], AmountJson);
 /**
  * See http://api.taler.net/wallet.html#general
  */
diff --git a/extension/background/wallet.ts b/extension/background/wallet.ts
index 5f9bb65a6..bb4326ec5 100644
--- a/extension/background/wallet.ts
+++ b/extension/background/wallet.ts
@@ -24,10 +24,31 @@
 
 /// <reference path="../decl/urijs/URIjs.d.ts" />
 /// <reference path="../decl/chrome/chrome.d.ts" />
-'use strict';
+"use strict";
 
 
-interface AmountJson {
+class MyClass {
+  value: number;
+  fraction: number;
+  currency: string;
+}
+
+
+@Checkable.Class
+class AmountJson {
+  @Checkable.Number
+  value: number;
+
+  @Checkable.Number
+  fraction: number;
+
+  @Checkable.String
+  currency: string;
+
+  static check: (v: any) => AmountJson;
+}
+
+interface AmountJson_interface {
   value: number;
   fraction: number
   currency: string;
@@ -58,11 +79,11 @@ interface Offer {
 
 interface Contract {
   H_wire: string;
-  amount: AmountJson;
+  amount: AmountJson_interface;
   auditors: string[];
   expiry: string,
   locations: string[];
-  max_fee: AmountJson;
+  max_fee: AmountJson_interface;
   merchant: any;
   merchant_pub: string;
   mints: MintInfo[];
@@ -77,7 +98,7 @@ interface CoinPaySig {
   coin_pub: string;
   ub_sig: string;
   denom_pub: string;
-  f: AmountJson;
+  f: AmountJson_interface;
 }
 
 
@@ -181,8 +202,8 @@ function signDeposit(db: IDBDatabase,
  * @param allowedMints
  */
 function getPossibleMintCoins(db: IDBDatabase,
-                              paymentAmount: AmountJson,
-                              depositFeeLimit: AmountJson,
+                              paymentAmount: AmountJson_interface,
+                              depositFeeLimit: AmountJson_interface,
                               allowedMints: MintInfo[]): Promise<MintCoins> {
 
 
@@ -617,7 +638,7 @@ function updateMintFromUrl(db, baseUrl) {
 
 function getBalances(db): Promise<any> {
   function collectBalances(c: Db.Coin, byCurrency) {
-    let acc: AmountJson = byCurrency[c.currentAmount.currency];
+    let acc: AmountJson_interface = byCurrency[c.currentAmount.currency];
     if (!acc) {
       acc = Amount.getZero(c.currentAmount.currency).toJson();
     }
diff --git a/extension/manifest.json b/extension/manifest.json
index 478c2a9b0..7c0f295b4 100644
--- a/extension/manifest.json
+++ b/extension/manifest.json
@@ -48,6 +48,7 @@
       "background/db.js",
       "background/query.js",
       "background/messaging.js",
+      "background/checkable.js",
       "background/http.js",
       "background/wallet.js"
     ]
diff --git a/extension/tsconfig.json b/extension/tsconfig.json
index 2d166debd..5586d78bc 100644
--- a/extension/tsconfig.json
+++ b/extension/tsconfig.json
@@ -10,6 +10,7 @@
       "background/db.ts",
       "background/query.ts",
       "background/http.ts",
+      "background/checkable.ts",
       "background/messaging.ts",
       "lib/util.ts",
       "lib/polyfill-react.ts",