function merge(target, a, arrayMode) {
  if (a) {
    const keys = Object.keys(a); // will return an array of own properties
    const len = keys.length; //don't make it inline
    for (let i = 0; i < len; i++) {
      if (arrayMode === 'strict') {
        target[keys[i]] = [ a[keys[i]] ];
      } else {
        target[keys[i]] = a[keys[i]];
      }
    }
  }
}
Copy all the properties of a into b.
@param {*} target
@param {*} a