Translate cursor relative to nearest node, not SourceElement (prettie…

…r#1989)

* Add test demonstrating SourceElement-relative cursor translation

See prettier#1981 (comment)

* Translate cursor relative to nearest node, not SourceElement

This partially address prettier#1981

See prettier#1981 (comment)

* Add test demonstrating incorrect cursor translation

Since the `cursorOffset` option (introduced in prettier#1637) works by tracking
the cursor position relative to an AST node (rather than a CST token),
it can produce incorrect results.

See prettier#1981

index.js

@@ -125,7 +125,8 @@ function findSiblingAncestors(startNodeAndParents, endNodeAndParents) {
   };
 }
 
-function findNodeAtOffset(node, offset, parentNodes) {
+function findNodeAtOffset(node, offset, predicate, parentNodes) {
+  predicate = predicate || (() => true);
   parentNodes = parentNodes || [];
   const start = util.locStart(node);
   const end = util.locEnd(node);
@@ -134,14 +135,15 @@ function findNodeAtOffset(node, offset, parentNodes) {
       const childResult = findNodeAtOffset(
         childNode,
         offset,
+        predicate,
         [node].concat(parentNodes)
       );
       if (childResult) {
         return childResult;
       }
     }
 
-    if (isSourceElement(node)) {
+    if (predicate(node)) {
       return {
         node: node,
         parentNodes: parentNodes
@@ -199,8 +201,16 @@ function calculateRange(text, opts, ast) {
     }
   }
 
-  const startNodeAndParents = findNodeAtOffset(ast, startNonWhitespace);
-  const endNodeAndParents = findNodeAtOffset(ast, endNonWhitespace);
+  const startNodeAndParents = findNodeAtOffset(
+    ast,
+    startNonWhitespace,
+    isSourceElement
+  );
+  const endNodeAndParents = findNodeAtOffset(
+    ast,
+    endNonWhitespace,
+    isSourceElement
+  );
   const siblingAncestors = findSiblingAncestors(
     startNodeAndParents,
     endNodeAndParents

tests/cursor/jsfmt.spec.js

@@ -6,3 +6,19 @@ test("translates cursor correctly in basic case", () => {
     cursorOffset: 1
   });
 });
+
+test("positions cursor relative to closest node, not SourceElement", () => {
+  const code = "return         15";
+  expect(prettier.formatWithCursor(code, { cursorOffset: 15 })).toEqual({
+    formatted: "return 15;\n",
+    cursorOffset: 7
+  });
+});
+
+test("keeps cursor inside formatted node", () => {
+  const code = "return         15";
+  expect(prettier.formatWithCursor(code, { cursorOffset: 14 })).toEqual({
+    formatted: "return 15;\n",
+    cursorOffset: 14 // TODO fix this
+  });
+});