Fixes #3533 (#3544)

* Fixes #3533

packages/less/Gruntfile.js

@@ -190,9 +190,10 @@ module.exports = function(grunt) {
     // Handle async / await in Rollup build for tests
     // Remove this when Node 6 is no longer supported for the build/test process
     const nodeVersion = semver.major(process.versions.node);
+    const tsNodeRuntime = path.resolve(path.join('node_modules', '.bin', 'ts-node'));
     let scriptRuntime = 'node';
     if (nodeVersion < 8) {
-        scriptRuntime = path.resolve(path.join('node_modules', '.bin', 'ts-node'));
+        scriptRuntime = tsNodeRuntime;
     }
 
     // Project configuration.
@@ -228,7 +229,10 @@ module.exports = function(grunt) {
                 command: scriptRuntime + " build/rollup.js --browser --out=./tmp/browser/less.min.js"
             },
             test: {
-                command: "node test/index.js"
+                command: [
+                    tsNodeRuntime + " test/test-es6.ts",
+                    "node test/index.js"
+                ].join(' && ')
             },
             generatebrowser: {
                 command: 'node test/browser/generator/generate.js'

packages/less/src/less/index.js

@@ -88,5 +88,14 @@ export default (environment, fileManagers) => {
         }
     }
 
+    /**
+     * Some of the functions assume a `this` context of the API object,
+     * which causes it to fail when wrapped for ES6 imports.
+     * 
+     * An assumed `this` should be removed in the future.
+     */
+    initial.parse = initial.parse.bind(api);
+    initial.render = initial.render.bind(api);
+
     return api;
 };

packages/less/test/test-es6.ts

@@ -0,0 +1,17 @@
+// https://github.com/less/less.js/issues/3533
+console.log('Testing ES6 imports...')
+
+import less from '..';
+let lessRender = less.render;
+
+// then I call lessRender on something
+let y = lessRender(`
+body {
+    a: 1;
+    b: 2;
+    c: 30;
+    d: 4;
+}`, {sourceMap: {}},  function(error, output) {
+  if (error)
+    console.error(error)
+})