Remove digits representing irrelevant precision from a number with JavaScript

function formatNumber(n) {
  
  // in case of a string... ParseFloat it
  n = parseFloat(n)
  
  // Fool-proof case where n is "Not A Number"
  if(isNaN(n)){return null}

  // Negative number detection
  let N = Math.abs(n);
  let isNegative = N !== n;
  
  // The zero case
  if(N===0){
    return n
  }
  
  // Numbers which do not need much processing
  if(N>1){return +(n.toFixed(1))}
  
  // Lets process numbers by moving the decimal dot to the right
  // until the number is more than 1
  let i = 0;
  while (Math.floor(N) < 1) {
    let dotPos = (""+N).indexOf(".") + 1
    N = (""+N).replace(".","")
    N = parseFloat(N.slice(0,dotPos)+"."+N.slice(dotPos))
    i++;
  }

  // Re-add the negative sign
  if (isNegative) {
    N = -N;
  }
  
  // Now round and reposition the decimal dot
  return Math.round(N) / Math.pow(10, i);
}

// ============================================================== Test cases
let testCases = [
  { in: 0.0001, out: 0.0001 },
  { in: 0.2453535, out: 0.2 },
  { in: 0.55, out: 0.6 },
  { in: 0.055, out: 0.06 },
  { in: 0.0055, out: 0.006 },
  { in: 0.15, out: 0.2 },
  { in: 0.015, out: 0.02 },
  { in: 0.0015, out: 0.002 },
  { in: 0.25, out: 0.3 },
  { in: 0.025, out: 0.03 },
  { in: 0.0025, out: 0.003 },
  { in: 0.00412, out: 0.004 },
  { in: -0.0045, out: -0.004 },
  { in: -0.55, out: -0.5 },
  { in: -0.15, out: -0.1 },
  { in: -0.015, out: -0.01 },
  { in: -0.0105, out: -0.01 },
  { in: -0.010504, out: -0.01 },
  { in: 2, out: 2 },
  { in: 2.01, out: 2.0 },
  { in: 2.34567, out: 2.3 },
  { in: 0, out: 0 },
  { in: "0.012%", out: 0.01 },
  { in: "Hello", out: null }
];

testCases.forEach((item) => {
  let res = formatNumber(item.in);
  let consoleMgs = `Test case: ${JSON.stringify(item)}\n Result: ${res}\n ${(res == item.out)?"PASS":"FAIL"}`
  if (res == item.out) {
    console.log(consoleMgs);
  } else {
    console.error(consoleMgs);
  }
});

//N = N * 10;

console.log(0.55*10)
console.log(0.055*10)
console.log(0.0055*10)
console.log(0.00055*10)