diff --git a/js/tests/id_test.js b/js/tests/id_test.js
deleted file mode 100644
index 0ea9e96d..00000000
--- a/js/tests/id_test.js
+++ /dev/null
@@ -1,22 +0,0 @@
-export const tests = []
-const t = (f) => tests.push(f)
-
-// id is declared and is a function
-t(() => typeof id === 'function')
-
-// id take 1 argument
-t(() => id.length === 1)
-
-// id return numbers back
-t(() => id(5) === 5)
-
-// id return strings back
-t(() => id('pouet') === 'pouet')
-
-// id return itself, why not
-t(() => id(id) === id)
-
-// id return anything really
-t((_) => id(_) === _)
-
-Object.freeze(tests)
diff --git a/js/tests/get-length_test.js b/js/tests/mutable.js
similarity index 56%
rename from js/tests/get-length_test.js
rename to js/tests/mutable.js
index 2eea9f4f..6797a03c 100644
--- a/js/tests/get-length_test.js
+++ b/js/tests/mutable.js
@@ -1,12 +1,23 @@
 export const tests = []
 const t = (f) => tests.push(f)
 
+// id is declared and is a function
+t(() => typeof id === 'function')
+// id take 1 argument
+t(() => id.length === 1)
+// id return numbers back
+t(() => id(5) === 5)
+// id return strings back
+t(() => id('pouet') === 'pouet')
+// id return itself, why not
+t(() => id(id) === id)
+// id return anything really
+t((_) => id(_) === _)
+
 // handle simple array
 t(() => getLength([2, 42]) === 2)
-
 // handle mixed array
 t(() => getLength(['pouet', 4, true]) === 3)
-
 t(() => getLength(Array(100)) === 100) // handle holey array
 t(() => getLength('salut') === 5) // handle strings
 t(() => getLength([]) === 0) // handle empty arrays
diff --git a/subjects/id.en.md b/subjects/id.en.md
deleted file mode 100644
index beeaded0..00000000
--- a/subjects/id.en.md
+++ /dev/null
@@ -1,10 +0,0 @@
-## Id
-
-### Instructions
-
-Create an `id` function that takes one argument and return it
-
-
-### Notions
-
-- [https://nan-academy.github.io/js-training/examples/functions.js](https://nan-academy.github.io/js-training/examples/functions.js)
diff --git a/subjects/get-length.en.md b/subjects/mutable.en.md
similarity index 50%
rename from subjects/get-length.en.md
rename to subjects/mutable.en.md
index 5ab344b4..d5a6e5dd 100644
--- a/subjects/get-length.en.md
+++ b/subjects/mutable.en.md
@@ -1,12 +1,15 @@
-## Get Length
+## mutable
 
 ### Instructions
 
-Create a `getLength` function that takes an array or a string
-and return its length.
+Create the following functions:
+
+- `id` that takes one argument and return it
+- `getLength` that takes an array or a string and return its length
 
 
 ### Notions
 
+- [https://nan-academy.github.io/js-training/examples/functions.js](https://nan-academy.github.io/js-training/examples/functions.js)
 - [https://nan-academy.github.io/js-training/examples/data-structures.js](https://nan-academy.github.io/js-training/examples/data-structures.js)
 - [https://nan-academy.github.io/js-training/examples/get.js](https://nan-academy.github.io/js-training/examples/get.js)