parseIfStatement.js

pp.parseStatement = function (declaration, topLevel) {
  let node = this.startNode();
  
  if (this.match(tokenTypes._if)) {
    return this.parseIfStatement(node);
  } else {
    ...
  }
}

pp.parseIfStatement = function (node) {
  this.next();
  node.test = this.parseParenExpression();
  node.consequent = this.parseStatement(false);
  node.alternate = this.eat(tokenTypes._else) ? this.parseStatement(false) : null;
  return this.finishNode(node, "IfStatement");
};