toggleValueInArray.js

// traditional version
function toggleValueInArray1(val, arr) {
  var ix = arr.indexOf(val);
  if(ix === -1){
    // add to array
    arr.push(val);
  }
  else
  {
    // remove from array
    arr.splice(ix, 1);
  }
}

// functional version
let toggleValueInArray2 = arr => val => {
  var ix = arr.indexOf(val);
  if(ix === -1){
    // add to array
    arr.push(val);
  }
  else
  {
    // remove from array
    arr.splice(ix, 1);
  }
}

Usage of the traditional version

var arr = ["socks", "pants"];
toggleValueInArray1("socks", arr);
toggleValueInArray1("shirt", arr);

// arr is now ["pants", "shirt"]

Usage of the fp version

let arr = ["socks", "pants"];
let toggler = toggleValueInArray2(arr);
toggler("socks");
toggler("shirt");

// arr is now ["pants", "shirt"]