Passed the exam. . .

function permute(input) {
  var permArr = [],
  usedChars = [];
  function main(input){
    var i, ch;
    for (i = 0; i < input.length; i++) {
      ch = input.splice(i, 1)[0];
      usedChars.push(ch);
      if (input.length == 0) {
        permArr.push(usedChars.slice());
      }
      main(input);
      input.splice(i, 0, ch);
      usedChars.pop();
    }
    return permArr;
  }
  return main(input).join('\n');
};
console.log(permute(['foo','bar','hello','world']));

var arr = ['foo', 'bar', 'hello', 'world']; 

// 4 3 2 1 
var first = {}
var result = []; 
arr.forEach((first, idx, its) => {
    its.filter(e => e!==first).forEach((sec, idx, its) => {
        its.filter(e => e!==sec).forEach((third, idx, its)=> { 
            its.filter(e => e!=third).forEach((forth, idx, its) => {
                console.log(first, sec, third, forth); 
                result.push(first + sec + third + forth); 
            }); 
        });
    });
});

= = Crazy traversal. . .

You can use the directed graph method to get all the paths of four nodes

Stacks and Queues