Covered it all

Author: ZakiMohammed

README.md

@@ -1,3 +1,112 @@
 # JavaScript Rest Parameter
 
-Understanding the JavaScript rest parameter
+Understanding the JavaScript Rest Parameter.
+
+## Life without Rest
+
+JavaScript functions don't give a func about extra parameters, they simply ignore them.
+```
+function add(num1, num2) {
+    return num1 + num2
+}
+
+console.log(add(100, 200, 500)) // 500 is ignored, lol
+
+// 300
+```
+
+One way to remember the forgotten is to use arguments keyword within a function.
+```
+function add(num1, num2) {
+    return num1 + num2 + arguments[2]
+}
+
+console.log(add(100, 200, 500))
+
+// 800
+
+// [Arguments] { '0': 100, '1': 200, '2': 500 }
+```
+
+But they doesn't jam with arrow arrow functions
+```
+const add = (num1, num2) => {
+    return num1 + num2 + arguments[2]
+}
+
+console.log(add(100, 200, 500))
+
+// 300[object Object]
+
+// arguments returns root level object 
+// (for browser it is window, for Node.js it is module wrapper variables)
+```
+
+The fall of arguments keyword:
+
+- it is an object acting as an array
+- provide all parameters (given or not given)
+- doesn't jam with arrow arrow functions
+- not an array, so can't use array functions
+- syntactically weirdo
+
+## Rest to the rescue
+
+Put 3 dots before a function's parameter and call it a rest parameter.
+```
+function add(...numbers) {
+    let total = 0
+    for (const num of numbers) {
+        total += num
+    }
+    return total
+}
+
+console.log(add(100, 200, 500))
+
+// 800
+```
+
+Leveraging the power of array functions with rest parameter.
+```
+const add = (...numbers) => numbers.reduce((acc, crr) => acc + crr, 0)
+
+console.log(add(100, 200, 500))
+
+// 800
+```
+
+Can also jam with arrow functions.
+```
+const add = (...numbers) => numbers.reduce((acc, crr) => acc + crr, 0)
+
+console.log(add(100, 200, 500))
+
+// 800
+```
+
+Rest must be the last parameter in a function's parameter list.
+```
+const info = (name, age, ...skills, gender) => `${name} ${age} - ${skills.join(', ')}`
+
+console.log(info('John', 21, 'Python', 'Bootstrap', 'Sass'))
+
+// Uncaught SyntaxError: Rest parameter must be last formal parameter
+```
+
+Failed to do so will leads to error and error nobody wants and dev are not nobody.
+```
+const info = (name, age, ...skills, gender) => `${name} ${age} - ${skills.join(', ')}`
+
+console.log(info('John', 21, 'Python', 'Bootstrap', 'Sass'))
+
+// Uncaught SyntaxError: Rest parameter must be last formal parameter
+```
+
+Summary about rest parameter:
+
+- its simply an array
+- benefited with array functions
+- jam with arrow functions
+- must be the last parameter
+- syntactically awesome

arguments-1.js

@@ -0,0 +1,9 @@
+function add(num1, num2) {
+    return num1 + num2 + arguments[2]
+}
+
+console.log(add(100, 200, 500))
+
+// 800
+
+// [Arguments] { '0': 100, '1': 200, '2': 500 }

arguments-2.js

@@ -0,0 +1,10 @@
+const add = (num1, num2) => {
+    return num1 + num2 + arguments[2]
+}
+
+console.log(add(100, 200, 500))
+
+// 300[object Object]
+
+// arguments returns root level object 
+// (for browser it is window, for Node.js it is module wrapper variables)

default.js

@@ -0,0 +1,7 @@
+function add(num1, num2) {
+    return num1 + num2
+}
+
+console.log(add(100, 200, 500)) // 500 is ignored, lol
+
+// 300