My code

Exercises/1-callback.js

@@ -1,5 +1,10 @@
 'use strict';
 
-const iterate = (obj, callback) => null;
+const iterate = (object, callback) => {
+  for (const key in object) {
+    if (Object.prototype.hasOwnProperty.call(object, key))
+      callback(key, object[key], object);
+  }
+};
 
 module.exports = { iterate };

Exercises/2-closure.js

@@ -1,5 +1,5 @@
 'use strict';
 
-const store = x => null;
+const store = x => () => x;
 
 module.exports = { store };

Exercises/3-wrapper.js

@@ -1,5 +1,19 @@
 'use strict';
 
-const contract = (fn, ...types) => null;
+const contract = (fn, ...types) => (...args) => {
+  for (const index in args) {
+    const arg = args[index];
+    if (arg !== types[index](arg)) throw new TypeError(
+      `type of argument '${arg}': '${typeof arg}'
+      does not match '${types[index]}'!`);
+  }
+  const res = fn(...args);
+  if (res !== types[types.length - 1](res))
+    throw new TypeError(
+      `type of result '${res}': '${typeof res}'
+       does not match '${types[types.length - 1]}'!`
+    );
+  return res;
+};
 
 module.exports = { contract };