fix(purity): rework purity principle slide

Author: Antoine RICHARD

docs/markdown/02-nodes-elements-components/22-purity.md

@@ -8,20 +8,20 @@ All React components **must** act like **pure functions** with respect to their
 
 # Purity
 
-## Pure function
+## Impure function
+
+Do you know why this function is not pure ?
 
 ```typescript
-function sum(a, b) {
-  return a + b;
+let count = 0;
+
+function incrementCount(obj) {
+  count++;
+  obj.value = count;
+  return count;
 }
 ```
 
-<!-- .element: class="big-code" -->
-
-- always returns the same result for a given set of parameters
-- does not modify its parameters
-- does not have semantical side effects
-
 ##==##
 
 <!-- .slide: class="with-code" -->
@@ -30,15 +30,36 @@ function sum(a, b) {
 
 ## Impure function
 
+Do you know why this function is not pure ?
+
 ```typescript
-const fee = 0.5;
+let count = 0;
 
-function withdraw(account, x) {
-  account.total -= x + fee;
+function incrementCount(obj) {
+  count++;
+  obj.value = count;
+  return count;
+}
+```
+
+- depends on external variable context
+- creates side effect (updates external variable and argument)
+
+##==##
+
+<!-- .slide: class="with-code" -->
+
+# Purity
+
+## Pure function
+
+```typescript
+function sum(a, b) {
+  return a + b;
 }
 ```
 
 <!-- .element: class="big-code" -->
 
-- modifies its parameters
-- and/or depends on external context
+- always returns the same result for a given set of parameters
+- does not create side effects