combinations.js

const combine = (instr, outstr, index) => {
  
    for (var i = index; i < instr.length; i++) {
      // append the character
      outstr = outstr.concat(instr.charAt(i));
      
      //print the result
      console.log(outstr);
      
      // make a recursive call at i + 1
      combine(instr, outstr, i + 1);
      
      // remove the character added in the first step
      outstr = outstr.substr(0, outstr.length - 1);
    }  
};

combine("abc", "", 0); // a, ab, abc, ac, b, bc, c