index.js

import traverse from "@babel/traverse";
import * as t from "@babel/types";
import helpers from "./helpers";

function makePath(path) {
  const parts = [];

  for (; path.parentPath; path = path.parentPath) {
    parts.push(path.key);
    if (path.inList) parts.push(path.listKey);
  }

  return parts.reverse().join(".");
}

/**
 * Given a file AST for a given helper, get a bunch of metadata about it so that Babel can quickly render
 * the helper is whatever context it is needed in.
 */
function getHelperMetadata(file) {
  const globals = new Set();
  const localBindingNames = new Set();
  // Maps imported identifier -> helper name
  const dependencies = new Map();

  let exportName;
  let exportPath;
  const exportBindingAssignments = [];
  const importPaths = [];
  const importBindingsReferences = [];

  traverse(file, {
    ImportDeclaration(child) {
      const name = child.node.source.value;
      if (!helpers[name]) {
        throw child.buildCodeFrameError(`Unknown helper ${name}`);
      }
      if (
        child.get("specifiers").length !== 1 ||
        !child.get("specifiers.0").isImportDefaultSpecifier()
      ) {
        throw child.buildCodeFrameError(
          "Helpers can only import a default value",
        );
      }
      const bindingIdentifier = child.node.specifiers[0].local;
      dependencies.set(bindingIdentifier, name);
      importPaths.push(makePath(child));
    },
    ExportDefaultDeclaration(child) {
      const decl = child.get("declaration");

      if (decl.isFunctionDeclaration()) {
        if (!decl.node.id) {
          throw decl.buildCodeFrameError(
            "Helpers should give names to their exported func declaration",
          );
        }

        exportName = decl.node.id.name;
      }
      exportPath = makePath(child);
    },
    ExportAllDeclaration(child) {
      throw child.buildCodeFrameError("Helpers can only export default");
    },
    ExportNamedDeclaration(child) {
      throw child.buildCodeFrameError("Helpers can only export default");
    },
    Statement(child) {
      if (child.isModuleDeclaration()) return;

      child.skip();
    },
  });

  traverse(file, {
    Program(path) {
      const bindings = path.scope.getAllBindings();

      Object.keys(bindings).forEach(name => {
        if (name === exportName) return;
        if (dependencies.has(bindings[name].identifier)) return;

        localBindingNames.add(name);
      });
    },
    ReferencedIdentifier(child) {
      const name = child.node.name;
      const binding = child.scope.getBinding(name, /* noGlobal */ true);
      if (!binding) {
        globals.add(name);
      } else if (dependencies.has(binding.identifier)) {
        importBindingsReferences.push(makePath(child));
      }
    },
    AssignmentExpression(child) {
      const left = child.get("left");

      if (!(exportName in left.getBindingIdentifiers())) return;

      if (!left.isIdentifier()) {
        throw left.buildCodeFrameError(
          "Only simple assignments to exports are allowed in helpers",
        );
      }

      const binding = child.scope.getBinding(exportName);

      if (binding && binding.scope.path.isProgram()) {
        exportBindingAssignments.push(makePath(child));
      }
    },
  });

  if (!exportPath) throw new Error("Helpers must default-export something.");

  // Process these in reverse so that mutating the references does not invalidate any later paths in
  // the list.
  exportBindingAssignments.reverse();

  return {
    globals: Array.from(globals),
    localBindingNames: Array.from(localBindingNames),
    dependencies,
    exportBindingAssignments,
    exportPath,
    exportName,
    importBindingsReferences,
    importPaths,
  };
}

/**
 * Given a helper AST and information about how it will be used, update the AST to match the usage.
 */
function permuteHelperAST(file, metadata, id, localBindings, getDependency) {
  if (localBindings && !id) {
    throw new Error("Unexpected local bindings for module-based helpers.");
  }

  if (!id) return;

  const {
    localBindingNames,
    dependencies,
    exportBindingAssignments,
    exportPath,
    exportName,
    importBindingsReferences,
    importPaths,
  } = metadata;

  const dependenciesRefs = {};
  dependencies.forEach((name, id) => {
    dependenciesRefs[id.name] =
      (typeof getDependency === "function" && getDependency(name)) || id;
  });

  const toRename = {};
  const bindings = new Set(localBindings || []);
  localBindingNames.forEach(name => {
    let newName = name;
    while (bindings.has(newName)) newName = "_" + newName;

    if (newName !== name) toRename[name] = newName;
  });

  if (id.type === "Identifier" && exportName !== id.name) {
    toRename[exportName] = id.name;
  }

  traverse(file, {
    Program(path) {
      // We need to compute these in advance because removing nodes would
      // invalidate the paths.
      const exp = path.get(exportPath);
      const imps = importPaths.map(p => path.get(p));
      const impsBindingRefs = importBindingsReferences.map(p => path.get(p));

      const decl = exp.get("declaration");
      if (id.type === "Identifier") {
        if (decl.isFunctionDeclaration()) {
          exp.replaceWith(decl);
        } else {
          exp.replaceWith(
            t.variableDeclaration("var", [t.variableDeclarator(id, decl.node)]),
          );
        }
      } else if (id.type === "MemberExpression") {
        if (decl.isFunctionDeclaration()) {
          exportBindingAssignments.forEach(assignPath => {
            const assign = path.get(assignPath);
            assign.replaceWith(t.assignmentExpression("=", id, assign.node));
          });
          exp.replaceWith(decl);
          path.pushContainer(
            "body",
            t.expressionStatement(
              t.assignmentExpression("=", id, t.identifier(exportName)),
            ),
          );
        } else {
          exp.replaceWith(
            t.expressionStatement(t.assignmentExpression("=", id, decl.node)),
          );
        }
      } else {
        throw new Error("Unexpected helper format.");
      }

      Object.keys(toRename).forEach(name => {
        path.scope.rename(name, toRename[name]);
      });

      for (const path of imps) path.remove();
      for (const path of impsBindingRefs) {
        const node = t.cloneNode(dependenciesRefs[path.node.name]);
        path.replaceWith(node);
      }

      // We only use "traverse" for all the handy scoping helpers, so we can stop immediately without
      // actually doing the traversal.
      path.stop();
    },
  });
}

const helperData = Object.create(null);
function loadHelper(name) {
  if (!helperData[name]) {
    const helper = helpers[name];
    if (!helper) {
      throw Object.assign(new ReferenceError(`Unknown helper ${name}`), {
        code: "BABEL_HELPER_UNKNOWN",
        helper: name,
      });
    }

    const fn = () => {
      return t.file(helper.ast());
    };

    const metadata = getHelperMetadata(fn());

    helperData[name] = {
      build(getDependency, id, localBindings) {
        const file = fn();
        permuteHelperAST(file, metadata, id, localBindings, getDependency);

        return {
          nodes: file.program.body,
          globals: metadata.globals,
        };
      },
      minVersion() {
        return helper.minVersion;
      },
      dependencies: metadata.dependencies,
    };
  }

  return helperData[name];
}

export function get(
  name,
  getDependency?: string => ?t.Expression,
  id?,
  localBindings?: string[],
) {
  return loadHelper(name).build(getDependency, id, localBindings);
}

export function minVersion(name: string) {
  return loadHelper(name).minVersion();
}

export function getDependencies(name: string): $ReadOnlyArray<string> {
  return Array.from(loadHelper(name).dependencies.values());
}

export function ensure(name: string) {
  loadHelper(name);
}

export const list = Object.keys(helpers)
  .map(name => name.replace(/^_/, ""))
  .filter(name => name !== "__esModule");

export default get;