How to implement a Fibonacci sequence in JavaScript?

Change number1 and number2 to global variables, and get rid of the loop.

let number1 = 0;
let number2 = 1;

function helperFibonacci() {
  let current = number1 + number2;
  number1 = number2;
  number2 = current;
  console.log(current);
}

for (let i = 0; i < 10; i++) {
  helperFibonacci();
}

Try with this one , works great , have a good coding :)

 function fib(n) {
 let fibList = [];
 if (n < 2) {
    fibList.push(0);
 } else {
    let prev = 0;
    let curr = 1;
    fibList.push(prev, curr);
    for (let i = 2; i < n; i++) {
        const next = prev + curr;
        prev = curr;
        curr = next;
        fibList.push(curr);
    }
 }
 console.log(fibList);
 }

OPTION 1 USING GENERATOR FUNCTIONS

I think the proper way to do this is to use a Generator function. Theese are functions meant to return sequences, they are built for this kinds of situations where you have a sequence that you can calculate based on a formula and want to get the next value at will.

So lets say you have a simple formula to calculate the nth fibonacci number

function fibonacci(n = 0) {
    if (n < 0){ throw 'Fibonacci not defined for negative numbers'} 
    if (n < 2) { return n};
    return fibonacci(n - 1) + fibonacci(n - 2); 
}

so then you just wrap it in a generator function (using the function* syntax, note the *)

function* fibonacciGenerator() {
  var index = 0;
  while (true){
        // yields the next fibonacci
        yield fibonacci(index++);
    }
}

Then you can use it creating a new generator and calling the next function to retrieve the next element in the sequence. Note the while true here represents that the sequence is infinite, that is that it never ends and I can keep getting more and more elements. This does not become an infinite loop because of the yield keyword

let fibonacciSequence = fibonacciGenerator();
fibonacciSequence.next().value; // 0
fibonacciSequence.next().value; // 1
fibonacciSequence.next().value; // 1
fibonacciSequence.next().value; // 2

You can read more about generator functions here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*

NOT USING GENERATOR FUNCTIONS

If you are not familiar with generator functions or cannot use them, you can do it with a regular function and save the "state". I dont recommend at all saving it in a global variable because its a considered bad practice. But you can create a closure.

Closures are like functions with their own scope, or environment where they can have variables.

function fibonacciGenerator() {
    var index = 0;
    return function () {
        return fibonacci(index++);
    }
}

then use it

let fibonacciSequence = fibonacciGenerator();
fibonacciSequence(); // 0
fibonacciSequence(); // 1
fibonacciSequence(); // 1
fibonacciSequence(); // 2

note that the structure is similar than that of generators, but we have normal functions instead.

You can read more about closures here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures

This will return first 10 fibos , as we passed 10 in pramas .

const fiboFun = (length) => {
  let fibos = [];

  for (let i = 0; i < length; i++) {
    if (fibos[i - 2] && fibos[i - 1]) {
      let add = fibos[i - 2] + fibos[i - 1];
      fibos.push(add);
    } else {
      fibos.push(i);
    }
  }
  return fibos;
};

console.log(fiboFun(10));

handling it from the most basic level(basics) :

function fibonacciGenerator (n) {
    let output = []
    if(n === 1){
       output.push(0) 
    }
    else if(n === 2){
        output.push(0)
        output.push(1)
    }
    else if(n > 2){
        output.push(0);
        output.push(1)
        for(let num = 0;num <= n - 3; num++){
            const new_number = output[num] + output[num+1]
            output.push(new_number)
        }
    }
    return output

}
console.log(fibonacciGenerator (4))