Formatter: Fix heredoc formatting (#1142)

Fixes #476

This PR addresses the heredoc formatting problem described in the issue.

---------

Co-authored-by: Marco Roth <marco.roth@intergga.ch>

Author: citizen428

javascript/packages/formatter/src/format-printer.ts

@@ -285,10 +285,17 @@ export class FormatPrinter extends Printer {
 
   /**
    * Format ERB content with proper spacing around the inner content.
-   * Returns empty string if content is empty, otherwise wraps content with single spaces.
+   * Returns empty string if content is empty, otherwise adds a leading space
+   * and a trailing space (or newline for heredoc content starting with "<<").
    */
   private formatERBContent(content: string): string {
-    return content.trim() ? ` ${content.trim()} ` : ""
+    let trimmedContent = content.trim();
+
+    // See: https://github.com/marcoroth/herb/issues/476
+    // TODO: revisit once we have access to Prism nodes
+    let suffix = trimmedContent.startsWith("<<") ? "\n" : " "
+
+    return trimmedContent ? ` ${trimmedContent}${suffix}` : ""
   }
 
   /**

javascript/packages/formatter/test/erb/erb.test.ts

@@ -1578,6 +1578,144 @@ describe("@herb-tools/formatter", () => {
     expect(result).toBe(expected)
   })
 
+  test("ERB output with heredoc (issue 476)", () => {
+    const input = dedent`
+      <%= <<EXAMPLE
+      example
+      EXAMPLE
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with squiggly heredoc", () => {
+    const input = dedent`
+      <%= <<~HEREDOC
+        example
+      HEREDOC
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with hyphen heredoc", () => {
+    const input = dedent`
+      <%= <<-HEREDOC
+      example
+      HEREDOC
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with single-quoted heredoc", () => {
+    const input = dedent`
+      <%= <<'HEREDOC'
+      no #{interpolation}
+      HEREDOC
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with double-quoted heredoc", () => {
+    const input = dedent`
+      <%= <<"HEREDOC"
+      with #{interpolation}
+      HEREDOC
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with heredoc and method chaining", () => {
+    const input = dedent`
+      <%= <<HEREDOC.strip
+      example
+      HEREDOC
+      %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with bitshift operator", () => {
+    const input = `<%= 1 << 4 %>`
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  test("ERB output with append operator", () => {
+    const input = `<%= array << item %>`
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(input)
+  })
+
+  // The heredoc is not detected because it doesn't start with "<<".
+  // See: https://github.com/marcoroth/herb/issues/476
+  // TODO: revisit once we have access to Prism nodes
+  test("ERB output with heredoc as method argument", () => {
+    const input = dedent`
+      <%= foo(<<HEREDOC)
+      example
+      HEREDOC
+      %>
+    `
+
+    const expected = dedent`
+      <%= foo(<<HEREDOC)
+      example
+      HEREDOC %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(expected)
+  })
+
+  // The heredoc is not detected because it doesn't start with "<<".
+  // See: https://github.com/marcoroth/herb/issues/476
+  // TODO: revisit once we have access to Prism nodes
+  test("ERB output with heredoc after assignment", () => {
+    const input = dedent`
+      <%= x = <<HEREDOC
+      example
+      HEREDOC
+      %>
+    `
+
+    const expected = dedent`
+      <%= x = <<HEREDOC
+      example
+      HEREDOC %>
+    `
+
+    const result = formatter.format(input)
+
+    expect(result).toBe(expected)
+  })
+
   test("keeps hyphen-attached inline element together during line wrapping", () => {
     const input = dedent`
       <div>