code: function isNoOperandBefore(string, index) {
    const before = string.substring(0, index).trim();
  
    return before.length === 0 || before.search(/[({,]&/) !== -1;
  }
docs: Checks if there is no operand before the currenc char
  In other words, the current char is at the start of a possible operation,
  e.g. at the string start, after the opening paren or after a comma
  
  @param {String} string - the input string
  @param {Number} index - current char's position in string
  @return {Boolean}