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Generate the Fibonacci sequence in JavaScript

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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Problem

The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones, starting with 0 and 1. To generate the Fibonacci sequence using code, you can use either a recursive or an iterative approach.
The simplest way to calculate the Fibonacci sequence is to use an iterative approach. You can use a for loop to generate the sequence up to the nth term, using an array to store the values.
The recursive approach is more elegant and concise, but it can be less efficient due to the overhead of function calls. Instead of a loop, it uses a function that calls itself with a smaller input until it reaches the base case of either n = 1 or n = 2, at which point it returns the corresponding array.

Solution

const fibonacci = n => {
  let fib = [];
  for (let i = 0; i < n; i++) {
    if (i <= 1) fib.push(i);
    else fib.push(fib[i - 1] + fib[i - 2]);
  }
  return fib;
};

fibonacci(6); // [0, 1, 1, 2, 3, 5]

The recursive approach is more elegant and concise, but it can be less efficient due to the overhead of function calls. Instead of a loop, it uses a function that calls itself with a smaller input until it reaches the base case of either n = 1 or n = 2, at which point it returns the corresponding array.

Code Snippets

const fibonacci = n => {
  let fib = [];
  for (let i = 0; i < n; i++) {
    if (i <= 1) fib.push(i);
    else fib.push(fib[i - 1] + fib[i - 2]);
  }
  return fib;
};

fibonacci(6); // [0, 1, 1, 2, 3, 5]

const fibonacci = n => {
  if (n === 1) return [0];
  if (n === 2) return [0, 1];
  const fib = fibonacci(n - 1);
  return fib.concat(fib[fib.length - 1] + fib[fib.length - 2]);
};

fibonacci(6); // [0, 1, 1, 2, 3, 5]

Context

From 30-seconds-of-code: fibonacci

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