diff --git a/changelog_unreleased/javascript/12349.md b/changelog_unreleased/javascript/12349.md
new file mode 100644
index 000000000..35bf9cba2
--- /dev/null
+++ b/changelog_unreleased/javascript/12349.md
@@ -0,0 +1,23 @@
+#### Fix idempotence issues caused by line comments after `=` in assignments (#12349 by @thorn0)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const kochabCooieGameOnOboleUnweave = // !!!
+  rhubarbRhubarb(annularCooeedSplicesWalksWayWay);
+
+// Prettier stable, first format
+const kochabCooieGameOnOboleUnweave = rhubarbRhubarb( // !!!
+  annularCooeedSplicesWalksWayWay
+);
+
+// Prettier stable, second format
+const kochabCooieGameOnOboleUnweave = rhubarbRhubarb(
+  // !!!
+  annularCooeedSplicesWalksWayWay
+);
+
+// Prettier main
+const kochabCooieGameOnOboleUnweave = // !!!
+  rhubarbRhubarb(annularCooeedSplicesWalksWayWay);
+```
diff --git a/src/language-js/print/assignment.js b/src/language-js/print/assignment.js
index 1cb8bc81c..972c2f58a 100644
--- a/src/language-js/print/assignment.js
+++ b/src/language-js/print/assignment.js
@@ -2,7 +2,7 @@
 
 const { isNonEmptyArray, getStringWidth } = require("../../common/util.js");
 const {
-  builders: { line, group, indent, indentIfBreak },
+  builders: { line, group, indent, indentIfBreak, lineSuffixBoundary },
   utils: { cleanDoc, willBreak, canBreak },
 } = require("../../document/index.js");
 const {
@@ -50,6 +50,7 @@ function printAssignment(
         group(leftDoc),
         operator,
         group(indent(line), { id: groupId }),
+        lineSuffixBoundary,
         indentIfBreak(rightDoc, { groupId }),
       ]);
     }
diff --git a/tests/format/js/assignment-comments/__snapshots__/jsfmt.spec.js.snap b/tests/format/js/assignment-comments/__snapshots__/jsfmt.spec.js.snap
index 5bac54654..84351ac84 100644
--- a/tests/format/js/assignment-comments/__snapshots__/jsfmt.spec.js.snap
+++ b/tests/format/js/assignment-comments/__snapshots__/jsfmt.spec.js.snap
@@ -1,5 +1,47 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`call.js format 1`] = `
+====================================options=====================================
+parsers: ["babel", "flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+if (true)
+  if (true)
+    if (true)
+      if (true)
+        if (true)
+          longVariableName1 = // @ts-ignore
+          (variable01 + veryLongVariableNameNumber2).method();
+
+=====================================output=====================================
+if (true)
+  if (true)
+    if (true)
+      if (true)
+        if (true)
+          longVariableName1 = // @ts-ignore
+            (variable01 + veryLongVariableNameNumber2).method();
+
+================================================================================
+`;
+
+exports[`call2.js format 1`] = `
+====================================options=====================================
+parsers: ["babel", "flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+const kochabCooieGameOnOboleUnweave = // ???
+      rhubarbRhubarb(annularCooeedSplicesWalksWayWay);
+
+=====================================output=====================================
+const kochabCooieGameOnOboleUnweave = // ???
+  rhubarbRhubarb(annularCooeedSplicesWalksWayWay);
+
+================================================================================
+`;
+
 exports[`function.js format 1`] = `
 ====================================options=====================================
 parsers: ["babel", "flow", "typescript"]
@@ -82,7 +124,8 @@ f3 = (
   a = b //comment
 ) => {};
 
-f4 = () => {}; // Comment
+f4 = // Comment
+  () => {};
 
 f5 =
   // Comment
@@ -109,7 +152,8 @@ let f3 = (
   a = b //comment
 ) => {};
 
-let f4 = () => {}; // Comment
+let f4 = // Comment
+  () => {};
 
 let f5 =
   // Comment
@@ -141,7 +185,8 @@ const bifornCringerMoshedPerplexSawder = // !!!
   anodyneCondosMalateOverateRetinol;
 
 =====================================output=====================================
-const kochabCooieGameOnOboleUnweave = annularCooeedSplicesWalksWayWay; // ???
+const kochabCooieGameOnOboleUnweave = // ???
+  annularCooeedSplicesWalksWayWay;
 
 const bifornCringerMoshedPerplexSawder = // !!!
   glimseGlyphsHazardNoopsTieTie +
diff --git a/tests/format/js/assignment-comments/call.js b/tests/format/js/assignment-comments/call.js
new file mode 100644
index 000000000..e78303905
--- /dev/null
+++ b/tests/format/js/assignment-comments/call.js
@@ -0,0 +1,7 @@
+if (true)
+  if (true)
+    if (true)
+      if (true)
+        if (true)
+          longVariableName1 = // @ts-ignore
+          (variable01 + veryLongVariableNameNumber2).method();
diff --git a/tests/format/js/assignment-comments/call2.js b/tests/format/js/assignment-comments/call2.js
new file mode 100644
index 000000000..e0956821e
--- /dev/null
+++ b/tests/format/js/assignment-comments/call2.js
@@ -0,0 +1,2 @@
+const kochabCooieGameOnOboleUnweave = // ???
+      rhubarbRhubarb(annularCooeedSplicesWalksWayWay);