internal/tsetse/rules/ban_mutable_exports_rule.ts

/**
 * @fileoverview Bans 'export' of mutable variables.
 * It is illegal to mutate them, so you might as well use 'const'.
 */

import * as ts from 'typescript';

import {Checker} from '../checker';
import {ErrorCode} from '../error_code';
import {AbstractRule} from '../rule';

export class Rule extends AbstractRule {
  readonly ruleName = 'ban-mutable-exports';
  readonly code = ErrorCode.BAN_MUTABLE_EXPORTS;

  register(checker: Checker) {
    // Strategy: take all the exports of the file, then look at whether
    // they're const or not.  This is simpler than the alternative of
    // trying to match all the various kinds of 'export' syntax, such
    // as 'export var ...', 'export {...}', 'export default ...'.
    checker.on(ts.SyntaxKind.SourceFile, checkFile, this.code);
  }
}

function checkFile(checker: Checker, file: ts.SourceFile) {
  // Allow in d.ts files, which are modelling external JS that doesn't
  // follow our rules.
  if (file.fileName.endsWith('.d.ts')) {
    return;
  }
  const sym = checker.typeChecker.getSymbolAtLocation(file);
  if (!sym) return;
  const exports = checker.typeChecker.getExportsOfModule(sym);
  for (const exp of exports) {
    // In the case of
    //   let x = 3; export {x};
    // The exported symbol is the latter x, but we must dealias to
    // the former to judge whether it's const or not.
    let sym = exp;
    if (sym.flags & ts.SymbolFlags.Alias) {
      sym = checker.typeChecker.getAliasedSymbol(exp);
    }
    const decl = sym.valueDeclaration;
    if (!decl) continue;  // Skip e.g. type declarations.

    if (decl.getSourceFile() !== file) {
      // Reexports are best warned about in the original file
      continue;
    }

    if (!ts.isVariableDeclaration(decl) && !ts.isBindingElement(decl)) {
      // Skip e.g. class declarations.
      continue;
    }

    const isConst = (ts.getCombinedNodeFlags(decl) & ts.NodeFlags.Const) !== 0;
    if (!isConst) {
      // Note: show the failure at the exported symbol's declaration site,
      // not the dealiased 'sym', so that the error message shows at the
      // 'export' statement and not the variable declaration.
      checker.addFailureAtNode(
          exp.declarations[0],
          `Exports must be const.`);
    }
  }
}