RECURSION

1. Use Recursion to Create a Range of Numbers, FAQ

1 Use Recursion to Create a Range of Numbers, FAQ

1.1 Code

function rangeOfNumbers(startNum, endNum) {
   if(startNum === endNum) {  
     return [endNum]; 
   } else {  
     let arr =  rangeOfNumbers(startNum + 1, endNum); 
     arr.push(startNum); 
     return arr;     
   }
}

let result = rangeOfNumbers(1,4);
console.log(result);

[ 4, 3, 2, 1 ]

1.2 Step By Step

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Step 8

Step 9

Step 10

Step 11

Step 12

Step 13

Step 14

Step 15

Step 16

Step 17

Step 18

Step 19

Step 20

1.3 What happens when the base case is evaluated as false?

The function calls itself: Steps: 4,6,8

1.4 What happens when the base case is evaluated as true?

Returns an array: Step: 10

1.5 The Array, where does it come from?

From the base case: Step 10

1.6 Why can I push startNum to arr?

Because after the base case is evaluated as true, arr is an array: Step: 11

1.7 Why I'm Getting [4,3,2,1] ?

Because JavaScript uses a "stack" (LIFO, last in first out): Steps 10, 13, 16, 19

1.8 How can I get [1,2,3,4]?

Replace push with unshift

function rangeOfNumbers(startNum, endNum) {
   if(startNum === endNum) {  
     return [endNum]; 
   } else {  
     let arr =  rangeOfNumbers(startNum + 1, endNum); 
     arr.unshift(startNum);  // <-  here  
     return arr;     
   }
}

let result = rangeOfNumbers(1,4);
console.log(result);

[ 1, 2, 3, 4 ]