add some comments and jsdocs to ExportMap

src/ExportMap.js

@@ -23,9 +23,17 @@ let ts;
 
 const log = debug('eslint-plugin-import:ExportMap');
 
+/** @type {Map<string, ExportMap>} */
 const exportCache = new Map();
 const tsconfigCache = new Map();
 
+/*
+ * We have prototype methods and static methods
+ * ExportMap has 2 steps: preparation and rule evaluation
+ * The static methods use cache keys to run only once for each invocation of ESLint
+ * The prototype methods run once for each rule for each file
+ */
+
 export default class ExportMap {
   constructor(path) {
     this.path = path;
@@ -44,7 +52,7 @@ export default class ExportMap {
     this.imports = new Map();
     this.errors = [];
     /**
-     * type {'ambiguous' | 'Module' | 'Script'}
+     * @type {'ambiguous' | 'Module' | 'Script'}
      */
     this.parseGoal = 'ambiguous';
   }
@@ -296,14 +304,18 @@ function captureTomDoc(comments) {
 
 const supportedImportTypes = new Set(['ImportDefaultSpecifier', 'ImportNamespaceSpecifier']);
 
-ExportMap.get = function (source, context) {
+ExportMap.get = /** @returns {ExportMap} */ function (source, context) {
   const path = resolve(source, context);
   if (path == null) { return null; }
 
   return ExportMap.for(childContext(path, context));
 };
 
-ExportMap.for = function (context) {
+/**
+ * Returns an instance of ExportMap *for* the hash key from the context
+ * (file being linted + specifier)
+ */
+ExportMap.for = /** @returns {ExportMap} */ function (context) {
   const { path } = context;
 
   const cacheKey = context.cacheKey || hashObject(context).digest('hex');
@@ -359,7 +371,7 @@ ExportMap.for = function (context) {
   return exportMap;
 };
 
-ExportMap.parse = function (path, content, context) {
+ExportMap.parse = /** @returns {ExportMap} */ function (path, content, context) {
   const m = new ExportMap(path);
   const isEsModuleInteropTrue = isEsModuleInterop();