Сергей Соколов

let a = prompt("Enter a value", 10); // a, b, c дальше не меняются
let b = prompt("Enter b value", 10); // поэтому можно
let c = prompt("Enter c value", 10);  // const
let result;
let discr = (b * b ) - 4 * a * c;
let sqrtDiscr = Math.sqrt(discr); // при отрицат. discr здесь NaN
if(a === 0 && b === 0 && c === 0) { // проверяем уже после вычислений
  throw new Error("The values must be bigger than 0");
} else if(discr < 0 ){ // проверяем уже после вычисления sqrtDiscr
  throw new Error("This Equation have not solution");
} else if(discr === 0 ){
  console.log("This Equation have only 1 solution")
  result = (-b) / (2 * a);
} else if (discr > 0){
  console.log("This Equation have 2 solution");
  result = (- b + sqrtDiscr) / (2 * a) + ";     " + (- b - sqrtDiscr) / (2 * a);
}

console.log(result);

function permut8(arr, prepend) {
  var i, version, el, result = [];
  prepend = prepend || [];
  if(arr.length === 1) return [arr];
  for( i=0; i<arr.length; i++) {
    if( arr.length === 2) {
      result.push( prepend.concat( [arr[i], arr[(i+1)%2]] ));
    } else {
      version = arr.slice();
      el = version.splice(i,1);
      result = result.concat( permut8( version, prepend.concat(el)));
    }
  }  
  return result;
}

var test = permut8( 'abcd'.split('') );
test.map( e=>e.join(' ')).join("\n")
/*
a b c d
a b d c
a c b d
a c d b
a d b c
a d c b
b a c d
b a d c
b c a d
b c d a
b d a c
b d c a
c a b d
c a d b
c b a d
c b d a
c d a b
c d b a
d a b c
d a c b
d b a c
d b c a
d c a b
d c b a
*/

function nextLexInPlace(arr){
  var i, a = -1, b = -1;
  for( i = 0; i < arr.length-1; i++) if(arr[i] < arr[1+i]) a = i;
  if( !~a) return; // no more permutations
  for( i = a + 1; i < arr.length; i++) if(arr[a] < arr[i]) b = i;
  swap(arr, a, b);
  a++;
  b = arr.length - 1;
  while( a < b) swap(arr, a++, b--);
  return true;
}

function swap( arr, a, b) {
  var xx = arr[a];
  arr[a] = arr[b];
  arr[b] = xx;
}

function allMutations( source) {
  var result = [], arr = source.slice();
  result.push( arr.sort().slice());
  while( nextLexInPlace(arr)) result.push(arr.slice());
  return result;
}

var test = ['a','c','c']; JSON.stringify( allMutations(test))
// [["a","c","c"],["c","a","c"],["c","c","a"]]

function* permutator(arr) {
  var i, a, b;

  function swap( arr, a, b) {
		var xx = arr[a];
		arr[a] = arr[b];
		arr[b] = xx;
	}
  
  yield arr.slice();

  while(true) {
    a = -1, b = -1;
		for( i = 0; i < arr.length-1; i++) if(arr[i] < arr[1+i]) a = i;
		if( !~a) return;
		for( i = a + 1; i < arr.length; i++) if(arr[a] < arr[i]) b = i;
		swap(arr, a++, b);
		b = arr.length - 1;
		while( a < b) swap(arr, a++, b--);
		yield arr.slice();
  }
}

function allMutations( source) {
  var all = [], result, G = permutator(source.slice().sort());
  while(true) {
    result = G.next();
    if(result.done) break;
    all.push( result.value);
  }
  return all;
}

var test = ['a','c','c']; JSON.stringify( allMutations(test))
// [["a","c","c"],["c","a","c"],["c","c","a"]]