Fibonacci sequence javascript on a loop

Question

I'm studying and making exercise of the Fibonacci sequence and I don't understand where is the "i" value goes to?

function fibonacciGenerator (n) {
    
      var output = [];
  if (n === 1) {
    output = [0];
  }     
  else if (n === 2) {
      output = [0, 1];
  }
  else if (n === 0) {
      output = ["Please put in the number"];
  }
  else {
      output = [0, 1];
      for (var i = 2; i < n; i++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }
      
  }
    return output;  
    
  console.log(output); 
}

So the problem line was this loop

else {
      output = [0, 1];
      for (var i = 2; i < n; i++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }

so the code just understand by itself that output that are next to [0,1,...] is "i" right? Why can't i just put this line like this

else {
      output = [0, 1];
      for (var n = 2; n < output.length; n++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }

(BTW It said "Uncaught ReferenceError: i is not defined")

Answer 1

for (var n = 2; n < output.length; n++) {
      output.push(output[output.length - 2] + output[output.length - 1]);
  }

will not work because the for-loop condition depends on the array length's (n < output.length) and you are modifying the array length's inside the loop.

For a for-loop, you should use 'let' instead of 'var', like this

for (let i = 2; i < n; i++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }

Check this stackoverflow post that explains in detail the difference between var and let

Answer 2

The issue in your code is with the loop condition. You are using the same variable n for both the function parameter and the loop variable. This can lead to unexpected behavior. Additionally, the loop condition n < output.length will never be true because you are pushing elements into the output array, making its length always greater than 2.

else {
      output = [0, 1];
      for (var n = 2; n < output.length; n++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }

use the below code it is correct

else {
      output = [0, 1];
      for (var i = 2; i < n; i++) {
          output.push(output[output.length - 2] + output[output.length - 1]);
      }