diff --git a/subjects/block-chain.en.md b/subjects/block-chain.en.md
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+## Block Chain
+
+### Instructions
+
+Create a `blockChain` that create a block in your very own block chain.
+
+the function takes 2 arguments:
+- `data` any valid JSON data
+- `prev` the previous block, if no block are given it should use the 
+  genesis block: `{ index: 0, hash: '0' }`
+
+A block must have the following properties:
+- `index`
+- `hash` a computed hash using the concatenation of the `index`, `hash`
+  and stringified `data` and hashing all of it using the provided `hashCode`.
+- `data` the data (not encoded in JSON)
+- `prev` the previous block
+- `chain` a function that takes a new `data` and create the next block with it.
+
+
+### Examples
+
+```js
+const first = blockChain({ a: 1 })
+console.log(first.index) //           -> 1
+console.log(first.data) //            -> { a: 1 }
+console.log(first.prev) //            -> { index: 0, hash: "0" }
+console.log(first.hash) //            -> '1103f27'
+console.log(hashCode('10{"a":1}')) // -> '1103f27'
+
+const second = first.chain({ hello: 'world' })
+console.log(second.hash) //                           -> '18drvvc'
+console.log(hashCode('21103f27{"hello":"world"}')) // -> '18drvvc'
+
+const chain = second
+  .chain({ value: 4455 })
+  .chain({ some: 'data' })
+  .chain({ cool: 'stuff' })
+
+const fork = second
+  .chain({ value: 335 })
+  .chain({ some: 'data' })
+  .chain({ cool: 'stuff' })
+
+console.log(chain.hash) //  -> '1qr3qfs'
+console.log(fork.hash) //   -> '1x9gsc1'
+console.log(chain.index) // -> 5
+console.log(fork.index) //  -> 5
+```
+
+
+### Notions
+
+- [devdocs.io/javascript/global_objects/json/stringify](https://devdocs.io/javascript/global_objects/json/stringify)
+
+
+### Code provided
+
+```js
+const hashCode = str =>
+  (
+    [...str].reduce((h, c) => (h = (h << 5) - h + c.charCodeAt(0)) & h, 0) >>> 0
+  ).toString(36)
+```
diff --git a/subjects/collections.en.md b/subjects/collections.en.md
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+## Collections
+
+### Instructions
+
+Write a bunch of function to move from one type to another
+
+- `arrToSet` from `Array` to `Set`
+- `arrToStr` from `Array` to `String`
+- `setToArr` from `Set` to `Array`
+- `setToStr` from `Set` to `String`
+- `strToArr` from `String` to `Array`
+- `strToSet` from `String` to `Set`
+- `mapToObj` from `Map` to `Object`
+- `objToArr` from `Object` to `Array`
+- `objToMap` from `Object` to `Map`
+- `arrToObj` from `Array` to `Object`
+- `strToObj` from `String` to `Object`
+
+and finally write a function `superTypeOf` that unlike `typeof` return
+a different value for advanced types like `Map` and `Set`
+
+
+### Examples
+
+```js
+const str = 'hello'
+const arr = [ 1, 2, 1, 3 ]
+const obj = { x: 45, y: 75, radius: 24 }
+const set = new Set()
+const map = new Map()
+set.add(1)
+set.add(2)
+set.add(1)
+set.add(3)
+map.add('a', 1)
+map.add('b', 2)
+map.add(3, 'c')
+map.add(4, 'd')
+
+arrToSet(arr) // -> Set { 1, 2, 3 }
+arrToStr(arr) // -> '1213'
+setToArr(set) // -> [1, 2, 3]
+setToStr(set) // -> '123'
+strToArr(str) // -> ['h', 'e', 'l', 'l', 'o']
+strToSet(str) // -> Set { 'h', 'e', 'l', 'o' }
+mapToObj(map) // -> { a: 1, b: 2, '3': 'c', '4': 'd' }
+objToArr(obj) // -> [45, 75, 24]
+objToMap(obj) // -> Map { 'x' => 45, 'y' => 75, 'radius' => 24 }
+arrToObj(arr) // -> { '0': 1, '1': 2, '3': 1, '4': 3 }
+strToObj(str) // -> { '0': 'h', '1': 'e', '2': 'l', '3': 'l', '4': 'o' }
+
+superTypeOf(map) //         -> 'Map'
+superTypeOf(set) //         -> 'Set'
+superTypeOf(obj) //         -> 'Object'
+superTypeOf(str) //         -> 'String'
+superTypeOf(666) //         -> 'Number'
+superTypeOf(NaN) //         -> 'Number'
+superTypeOf(arr) //         -> 'Array'
+superTypeOf(null) //        -> 'null'
+superTypeOf(undefined) //   -> 'undefined'
+superTypeOf(superTypeOf) // -> 'Function'
+```
+
+
+### Notions
+
+- [devdocs.io/javascript/operators/typeof](https://devdocs.io/javascript/operators/typeof)
+- [devdocs.io/javascript/operators/spread_syntax](https://devdocs.io/javascript/operators/spread_syntax)
+- [devdocs.io/javascript/global_objects/map](https://devdocs.io/javascript/global_objects/map)
+- [devdocs.io/javascript/global_objects/set](https://devdocs.io/javascript/global_objects/set)
+- [devdocs.io/javascript/global_objects/object/fromentries](https://devdocs.io/javascript/global_objects/object/fromentries)
+- [devdocs.io/javascript/global_objects/object/entries](https://devdocs.io/javascript/global_objects/object/entries)