diff --git a/index.js b/index.js
index 1cfaa6274..4ab018d58 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var rlp = require('rlp');
 
 var Contract = require('./lib/contract.js');
diff --git a/lib/contract.js b/lib/contract.js
index 80949102a..617fb7e1b 100644
--- a/lib/contract.js
+++ b/lib/contract.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var utils = require('./utils.js');
 
 // Creates property that is immutable
@@ -291,6 +293,8 @@ function Interface(abi) {
     var methods = [], events = [];
     abi.forEach(function(method) {
 
+        var func = null;
+
         switch (method.type) {
             case 'function':
                 methods.push(method.name);
@@ -372,6 +376,7 @@ function Interface(abi) {
                 })();
                 break;
         }
+
         utils.defineProperty(this, method.name, func);
     }, this);
 
@@ -452,12 +457,12 @@ var allowedTransactionKeys = {
     data: true, from: true, gasLimit: true, gasPrice:true, to: true, value: true
 }
 
-function Contract(web3, wallet, contractAddress, interface) {
+function Contract(web3, wallet, contractAddress, contractInterface) {
     utils.defineProperty(this, 'web3', web3);
     utils.defineProperty(this, 'wallet', wallet);
     utils.defineProperty(this, 'contractAddress', contractAddress);
 
-    utils.defineProperty(this, 'interface', interface);
+    utils.defineProperty(this, 'interface', contractInterface);
 
     function getWeb3Promise(method) {
         var params = Array.prototype.slice.call(arguments, 1);
@@ -522,7 +527,7 @@ function Contract(web3, wallet, contractAddress, interface) {
             var transaction = {}
 
             var params = Array.prototype.slice.call(arguments);
-            if (params.length == interface[method].inputs.length + 1) {
+            if (params.length == contractInterface[method].inputs.length + 1) {
                 transaction = params.pop();
                 if (typeof(transaction) !== 'object') {
                     throw new Error('invalid transaction overrides');
@@ -534,7 +539,7 @@ function Contract(web3, wallet, contractAddress, interface) {
                 }
             }
 
-            var call = interface[method].apply(interface, params);
+            var call = contractInterface[method].apply(contractInterface, params);
             switch (call.type) {
                 case 'call':
                     ['data', 'gasLimit', 'gasPrice', 'to', 'value'].forEach(function(key) {
@@ -605,12 +610,12 @@ function Contract(web3, wallet, contractAddress, interface) {
         };
     }
 
-    interface.methods.forEach(function(method) {
+    contractInterface.methods.forEach(function(method) {
         utils.defineProperty(this, method, runMethod(method));
     }, this);
 
-    interface.events.forEach(function(method) {
-        var call = interface[method].apply(interface, []);
+    contractInterface.events.forEach(function(method) {
+        var call = contractInterface[method].apply(contractInterface, []);
         Object.defineProperty(self, 'on' + call.name.toLowerCase(), {
             enumerable: true,
             get: function() {
diff --git a/lib/secret-storage.js b/lib/secret-storage.js
index c92b902d4..95c553e0c 100644
--- a/lib/secret-storage.js
+++ b/lib/secret-storage.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var aes = require('aes-js');
 var pbkdf2 = require('pbkdf2');
 var scrypt = require('scrypt-js');
diff --git a/lib/signing-key.js b/lib/signing-key.js
index e59d0e0b9..b76f9c08f 100644
--- a/lib/signing-key.js
+++ b/lib/signing-key.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var elliptic = require('elliptic');
 
 var utils = require('./utils.js');
@@ -89,7 +91,7 @@ var ibanChecksum = (function() {
             expanded = parseInt(block, 10) % 97 + expanded.substring(block.length);
         }
 
-        checksum = String(98 - (parseInt(expanded, 10) % 97));
+        var checksum = String(98 - (parseInt(expanded, 10) % 97));
         while (checksum.length < 2) { checksum = '0' + checksum; }
 
         return checksum;
diff --git a/lib/utils.js b/lib/utils.js
index 214b689be..c73da6cf1 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -1,3 +1,4 @@
+'use strict';
 
 var BN = require('../node_modules/elliptic/node_modules/bn.js/lib/bn.js');
 var hash = require('../node_modules/elliptic/node_modules/hash.js/lib/hash.js');
diff --git a/lib/wallet.js b/lib/wallet.js
index 278fc5caa..e29271d5f 100644
--- a/lib/wallet.js
+++ b/lib/wallet.js
@@ -1,3 +1,5 @@
+'use strict';
+
 var rlp = require('rlp');
 
 var Contract = require('./contract.js');