Index an array by multiple fields

index-by.js

const indexBy = (value, ...fields) => {	
  if (fields.length === 0) {	
    return value;	
  }	
  const [currentField, ...rest] = fields;	
  const newValue = mapObjectValues(value, Array.isArray, indexByField, currentField);	
  return indexBy(newValue, ...rest);	
};	
Array.prototype.indexBy = function(...fields) {	
  return indexBy(this, ...fields);	
};