for-example.js

/**  input array  */
const arrInput = [1,4,2];

/** prepare an output array */
const arrOutput = new Array();

/** loop through the index of the input array */
for (var idx in arrInput) {
  /** add to the output array ... */
  arrOutput.push(
    /** ... the value of the input array plus 2 */
    arrInput[idx]+2
  )
}