enable esm support in Node v10

docs/index.md

@@ -1443,6 +1443,7 @@ More information can be found in the [Node.js documentation](https://nodejs.org/
 
 > Mocha supports ES modules only from Node.js v12.11.0 and above. To enable this in versions smaller than 13.2.0, you need to add `--experimental-modules` when running
 > Mocha. From version 13.2.0 of Node.js, you can use ES modules without any flags.
+> (Mocha _will_ load ESM even in Node v10, but this is not officially supported. Use at your own risk.)
 
 ### Current Limitations
 

lib/mocha.js

@@ -14,7 +14,9 @@ var utils = require('./utils');
 var mocharc = require('./mocharc.json');
 var errors = require('./errors');
 var Suite = require('./suite');
-var esmUtils = utils.supportsEsModules() ? require('./esm-utils') : undefined;
+var esmUtils = utils.supportsEsModules(false)
+  ? require('./esm-utils')
+  : undefined;
 var createStatsCollector = require('./stats-collector');
 var createInvalidReporterError = errors.createInvalidReporterError;
 var createInvalidInterfaceError = errors.createInvalidInterfaceError;

lib/utils.js

@@ -809,16 +809,20 @@ exports.defineConstants = function(obj) {
  * This function returns whether Node.JS has ES Module supports that is compatible with Mocha's needs,
  * which is version >=12.11.
  *
+ * @param {fullSpecification} whether the full Node.js ESM support is available (>= 12) or just something that supports the runtime (>= 10)
+ *
  * @returns {Boolean} whether the current version of Node.JS supports ES Modules in a way that is compatible with Mocha
  */
-exports.supportsEsModules = function() {
+exports.supportsEsModules = function(fullSpecification) {
   if (!process.browser && process.versions && process.versions.node) {
     var versionFields = process.versions.node.split('.');
     var major = +versionFields[0];
     var minor = +versionFields[1];
 
-    if (major >= 13 || (major === 12 && minor >= 11)) {
-      return true;
+    if (fullSpecification) {
+      return major >= 13 || (major === 12 && minor >= 11);
+    } else {
+      return major >= 10;
     }
   }
 };

test/integration/esm.spec.js

@@ -6,7 +6,7 @@ var args =
 
 describe('esm', function() {
   before(function() {
-    if (!utils.supportsEsModules()) this.skip();
+    if (!utils.supportsEsModules(true)) this.skip();
   });
 
   it('should pass a passing esm test that uses esm', function(done) {