[Answer] How should this JavaScript recursive function be understood?

[Answer] How should this JavaScript recursive function be understood?

Recursion means solving a repetitive problem by repeatedly calling the function itself. In this example, recursion is used to calculate the Fibonacci sequence, which is also a classic problem often solved with recursion.

This sample supports direct invocation with an input argument. If no Fibonacci parameter is provided when calling it, you need to enter a number from the command line and then perform the calculation.

// JavaScript command-line input/output
const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

// Fibonacci sequence function, recursive call
function fibo(index) {
  if (parseInt(index) === 1 || parseInt(index) === 2) {
    return 1;
  } else {
    return fibo(parseInt(index) - 1) + fibo(parseInt(index) - 2);
  }
}

// If a number is passed when invoking the script, compute it directly
if (process.argv.length > 2) {
  let index = parseInt(process.argv[2]); // argv[0] is node.exe, argv[1] is the filename
  console.log(`the fibonacci sequence of ${index} is ${fibo(index)}`);
  process.exit(0);
}

// If no number is provided when invoking, enter it here
console.log("please input an integer to calculate the fibonacci sequence");
rl.on('line', (index) => {
  console.log(`the fibonacci sequence of ${index} is ${fibo(index)}`);
  rl.close();
});