code: function parseArrayInitialiser() {
    var elements = [];
  
    index = lookahead.start;
    expect('[');
  
    while (!match(']')) {
      if (match(',')) {
        lex();
        elements.push(null);
      } else {
        elements.push(parseConditionalExpression());
  
        if (!match(']')) {
          expect(',');
        }
      }
    }
  
    lex();
  
    return finishArrayExpression(elements);
  }
docs: 11.1.4 Array Initialiser