index.js

import { declare } from "@babel/helper-plugin-utils";
import syntaxOptionalChaining from "@babel/plugin-syntax-optional-chaining";
import { types as t } from "@babel/core";

export default declare((api, options) => {
  api.assertVersion(7);

  const { loose = false } = options;

  function isSimpleMemberExpression(expression) {
    return (
      t.isIdentifier(expression) ||
      t.isSuper(expression) ||
      (t.isMemberExpression(expression) &&
        !expression.computed &&
        isSimpleMemberExpression(expression.object))
    );
  }

  return {
    name: "proposal-optional-chaining",
    inherits: syntaxOptionalChaining,

    visitor: {
      "OptionalCallExpression|OptionalMemberExpression"(path) {
        const { parentPath, scope } = path;
        let isDeleteOperation = false;
        const optionals = [];

        let optionalPath = path;
        while (
          optionalPath.isOptionalMemberExpression() ||
          optionalPath.isOptionalCallExpression()
        ) {
          const { node } = optionalPath;
          if (node.optional) {
            optionals.push(node);
          }

          if (optionalPath.isOptionalMemberExpression()) {
            optionalPath.node.type = "MemberExpression";
            optionalPath = optionalPath.get("object");
          } else if (optionalPath.isOptionalCallExpression()) {
            optionalPath.node.type = "CallExpression";
            optionalPath = optionalPath.get("callee");
          }
        }

        let replacementPath = path;
        if (parentPath.isUnaryExpression({ operator: "delete" })) {
          replacementPath = parentPath;
          isDeleteOperation = true;
        }
        for (let i = optionals.length - 1; i >= 0; i--) {
          const node = optionals[i];

          const isCall = t.isCallExpression(node);
          const replaceKey = isCall ? "callee" : "object";
          const chain = node[replaceKey];

          let ref;
          let check;
          if (loose && isCall && isSimpleMemberExpression(chain)) {
            // If we are using a loose transform (avoiding a Function#call) and we are at the call,
            // we can avoid a needless memoize. We only do this if the callee is a simple member
            // expression, to avoid multiple calls to nested call expressions.
            check = ref = chain;
          } else {
            ref = scope.maybeGenerateMemoised(chain);
            if (ref) {
              check = t.assignmentExpression(
                "=",
                t.cloneNode(ref),
                // Here `chain` MUST NOT be cloned because it could be updated
                // when generating the memoised context of a call espression
                chain,
              );
              node[replaceKey] = ref;
            } else {
              check = ref = chain;
            }
          }

          // Ensure call expressions have the proper `this`
          // `foo.bar()` has context `foo`.
          if (isCall && t.isMemberExpression(chain)) {
            if (loose && isSimpleMemberExpression(chain)) {
              // To avoid a Function#call, we can instead re-grab the property from the context object.
              // `a.?b.?()` translates roughly to `_a.b != null && _a.b()`
              node.callee = chain;
            } else {
              // Otherwise, we need to memoize the context object, and change the call into a Function#call.
              // `a.?b.?()` translates roughly to `(_b = _a.b) != null && _b.call(_a)`
              const { object } = chain;
              let context = scope.maybeGenerateMemoised(object);
              if (context) {
                chain.object = t.assignmentExpression("=", context, object);
              } else if (t.isSuper(object)) {
                context = t.thisExpression();
              } else {
                context = object;
              }

              node.arguments.unshift(t.cloneNode(context));
              node.callee = t.memberExpression(
                node.callee,
                t.identifier("call"),
              );
            }
          }

          replacementPath.replaceWith(
            t.conditionalExpression(
              loose
                ? t.binaryExpression("==", t.cloneNode(check), t.nullLiteral())
                : t.logicalExpression(
                    "||",
                    t.binaryExpression(
                      "===",
                      t.cloneNode(check),
                      t.nullLiteral(),
                    ),
                    t.binaryExpression(
                      "===",
                      t.cloneNode(ref),
                      scope.buildUndefinedNode(),
                    ),
                  ),
              isDeleteOperation
                ? t.booleanLiteral(true)
                : scope.buildUndefinedNode(),
              replacementPath.node,
            ),
          );

          replacementPath = replacementPath.get("alternate");
        }
      },
    },
  };
});