Repository: prettier/prettier (JavaScript) — Bug Fix

Issue:
Modification of nested arrays inside format string
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**Prettier 3.3.3**
[Playground link](https://prettier.io/playground/#N4Igxg9gdgLgprEAuEAPABAXnQAwCTADahAjADToBMAuhYQMwUAs11AvjiGSBAA4wBLaAGdkoAIYAnSRADuABSkJRKcQBtZ4gJ6juAI0niwAazgwAyuIC2cADICocZADN1wuPsMmz53kYcA5sgwkgCuHiDuVgLBYRFwqLxwkgI2sOoAKslQUgJwKq5q7tzCgWpwAIqhEPAubhEAVsKo5mWV1bVIhcUgAI4dcPIyvCog4sIAtI5wACazXCAh4gJqgQDCEFZW4shjamoLpVAB5QCCMCECeqHw8sn2jnVFEQAWMFZqAOovAvDCfmA4OZlL8BAA3X5aXZgYS6EBg8IASSgc1g5jAKX4pxR5hgWnKTx6vBk7k+hl4u2J+WSYKc3Ac7kkMCG4gC20JET8kkZuz04j0cAO3GJDhgnwEMxgL2QAA4AAzcSRwfoCJUstk7Lr1bgwfniyXSpCUbihdwZfkFbUgOBWAUzOYzWziY6hVlwABiEEk2wugV24huEBAbDYQA)

```sh
# Options (if any):
None
```

**Input:**

```jsx
x = `${[[1, 2], [3, 4]]}`
````

**Output:**

```jsx
x = `${[
  [2],
  [4],
]}`;
```

**Expected output:**

```jsx
x = `${[
  [1, 2],
  [3, 4],
]}`;
```

**Why?**

Prettier is changing the actual array contents, which it should not. Seems like it always takes the last element of each nested array.

I can work around it by pulling the array out into a separate variable ([playground link](https://prettier.io/playground/#N4Igxg9gdgLgprEAuEBDATugBAXiwbXwEYAaLAJgF0z8BmMgFksoB0oAPXLAAwBJgM6AL7cQJEBAAOMAJbQAzslCCIAdwAKGBIpSoANqtQBPReIBG6VGADWcGAGVUAWzgAZGVDjIAZvvlxzSxs7e0krDwBzZBh0AFcAkH8nGWi4hLh2STh0GRdYfQAVbKgMGTgdXz1-cXlIvTgARViIeB8-BIAreXZ7Osbm1qRK6pAARwG4dXQpHTR5AFpPOAATFbEQGNQZPUiAYQgnJ1RkND09ddqoCPqAQRgYmTNY+HVs9082qoSACxgnPQA6t8ZPB5GEwHB7NoQTIAG4gownMDyUwgWHxACSUFWsHsYBy0hu2PsMCM9U+I0k038AMskhOVPK2VhXnEHn86BgU1QESOFISYXQHJOZlQZjg53EVI8MABMmWMG+yAAHAAGcToODjGSa7m845DdriGBiuUKpVIcjiWL+ApiipGkBwJzi5arZauVBXWI8uAAMQg6CO90iJ1QzwgICEQiAA)).



Discussion:

Resolving patch:
diff --git a/changelog_unreleased/api/13315.md b/changelog_unreleased/api/13315.md
new file mode 100644
index 000000000000..cc593ea5cff3
--- /dev/null
+++ b/changelog_unreleased/api/13315.md
@@ -0,0 +1,3 @@
+#### Stop doc mutation in `prettier.doc.printDocToString` (#13315 by @fisker)
+
+For performance reason, `prettier.doc.printDocToString` used to mutate `.parts` of the [`fill`](https://github.com/prettier/prettier/blob/main/commands.md#fill) command during print. It was converted to a [pure function](https://en.wikipedia.org/wiki/Pure_function) to ensure output correctness.
diff --git a/changelog_unreleased/javascript/13315-2.md b/changelog_unreleased/javascript/13315-2.md
new file mode 100644
index 000000000000..3cfa3aff9459
--- /dev/null
+++ b/changelog_unreleased/javascript/13315-2.md
@@ -0,0 +1,19 @@
+#### Fix template literal print with array (#13315 by @fisker, @syi0808)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const string = `${[[1, 2], [3, 4]]}`
+
+// Prettier stable
+const string = `${[
+  [1, 2],
+  [3, 4],
+]}`;
+
+// Prettier main
+const string = `${[
+  [1, 2],
+  [3, 4],
+]}`;
+```
diff --git a/src/document/printer.js b/src/document/printer.js
index 552b44c39108..00449808006e 100644
--- a/src/document/printer.js
+++ b/src/document/printer.js
@@ -1,6 +1,6 @@
 import { convertEndOfLineToChars } from "../common/end-of-line.js";
 import getStringWidth from "../utils/get-string-width.js";
-import { fill, hardlineWithoutBreakParent, indent } from "./builders.js";
+import { hardlineWithoutBreakParent, indent } from "./builders.js";
 import {
   DOC_TYPE_ALIGN,
   DOC_TYPE_ARRAY,
@@ -32,6 +32,8 @@ const MODE_FLAT = Symbol("MODE_FLAT");
 
 const CURSOR_PLACEHOLDER = Symbol("cursor");
 
+const IS_MUTABLE_FILL = Symbol("IS_MUTABLE_FILL");
+
 function rootIndent() {
   return { value: "", length: 0, queue: [] };
 }
@@ -503,16 +505,26 @@ function printDocToString(doc, options) {
           break;
         }
 
+        const secondContent = parts[2];
+
         // At this point we've handled the first pair (context, separator)
-        // and will create a new fill doc for the rest of the content.
-        // Ideally we wouldn't mutate the array here but copying all the
-        // elements to a new array would make this algorithm quadratic,
+        // and will create a new *mutable* fill doc for the rest of the content.
+        // Copying all the elements to a new array would make this algorithm quadratic,
         // which is unusable for large arrays (e.g. large texts in JSX).
-        parts.splice(0, 2);
-        /** @type {Command} */
-        const remainingCmd = { ind, mode, doc: fill(parts) };
+        // https://github.com/prettier/prettier/issues/3263#issuecomment-344275152
+        let remainingDoc = doc;
+        if (doc[IS_MUTABLE_FILL]) {
+          parts.splice(0, 2);
+        } else {
+          remainingDoc = {
+            ...doc,
+            parts: parts.slice(2),
+            [IS_MUTABLE_FILL]: true,
+          };
+        }
 
-        const secondContent = parts[0];
+        /** @type {Command} */
+        const remainingCmd = { ind, mode, doc: remainingDoc };
 
         /** @type {Command} */
         const firstAndSecondContentFlatCmd = {


Test patch:
diff --git a/tests/format/js/template-literals/__snapshots__/format.test.js.snap b/tests/format/js/template-literals/__snapshots__/format.test.js.snap
index b635df2fd63b..d4470d98754e 100644
--- a/tests/format/js/template-literals/__snapshots__/format.test.js.snap
+++ b/tests/format/js/template-literals/__snapshots__/format.test.js.snap
@@ -164,6 +164,8 @@ descirbe('something', () => {
 
 throw new Error(\`pretty-format: Option "theme" has a key "\${key}" whose value "\${value}" is undefined in ansi-styles.\`,)
 
+a = \`\${[[1, 2, 3], [4, 5, 6]]}\`
+
 =====================================output=====================================
 const long1 = \`long \${
   a.b //comment
@@ -224,6 +226,11 @@ throw new Error(
   \`pretty-format: Option "theme" has a key "\${key}" whose value "\${value}" is undefined in ansi-styles.\`,
 );
 
+a = \`\${[
+  [1, 2, 3],
+  [4, 5, 6],
+]}\`;
+
 ================================================================================
 `;
 
diff --git a/tests/format/js/template-literals/expressions.js b/tests/format/js/template-literals/expressions.js
index c68f1176a1f3..c2f98b36b4cb 100644
--- a/tests/format/js/template-literals/expressions.js
+++ b/tests/format/js/template-literals/expressions.js
@@ -44,3 +44,5 @@ descirbe('something', () => {
 })
 
 throw new Error(`pretty-format: Option "theme" has a key "${key}" whose value "${value}" is undefined in ansi-styles.`,)
+
+a = `${[[1, 2, 3], [4, 5, 6]]}`
diff --git a/tests/unit/doc-printer.js b/tests/unit/doc-printer.js
new file mode 100644
index 000000000000..bf4dbf81d86d
--- /dev/null
+++ b/tests/unit/doc-printer.js
@@ -0,0 +1,45 @@
+// TODO: Find a better way to test the performance
+import { fill, join, line } from "../../src/document/builders.js";
+import { printDocToString } from "../../src/document/printer.js";
+
+test("`printDocToString` should not manipulate docs", () => {
+  const printOptions = { printWidth: 40, tabWidth: 2 };
+  const doc = fill(
+    join(
+      line,
+      Array.from({ length: 255 }, (_, index) => String(index + 1)),
+    ),
+  );
+
+  expect(doc.parts.length).toBe(255 + 254);
+
+  const { formatted: firstPrint } = printDocToString(doc, printOptions);
+
+  expect(doc.parts.length).toBe(255 + 254);
+
+  const { formatted: secondPrint } = printDocToString(doc, printOptions);
+
+  expect(firstPrint).toBe(secondPrint);
+
+  {
+    // About 1000 lines, #3263
+    const WORD = "word";
+    const hugeParts = join(
+      line,
+      Array.from(
+        { length: 1000 * Math.ceil(printOptions.printWidth / WORD.length) },
+        () => WORD,
+      ),
+    );
+    const orignalLength = hugeParts.length;
+
+    const startTime = performance.now();
+    const { formatted } = printDocToString(fill(hugeParts), printOptions);
+    const endTime = performance.now();
+    expect(hugeParts.length).toBe(orignalLength);
+
+    const lines = formatted.split("\n");
+    expect(lines.length).toBeGreaterThan(1000);
+    expect(endTime - startTime).toBeLessThan(1000);
+  }
+});