fix(formatter): preserve numeric separators in number literals

crates/oxc_formatter/src/formatter/token/number.rs

@@ -117,6 +117,14 @@ pub fn format_trimmed_number(text: &str, options: NumberFormatOptions) -> Cow<'_
     use FormatNumberLiteralState::{DecimalPart, Exponent, IntegerPart};
 
     let text = text.cow_to_ascii_lowercase();
+
+    // Bail out for numbers with numeric separators (underscores).
+    // Prettier's regex-based approach preserves them implicitly because `\d` doesn't match `_`.
+    // The only transformation that still applies is adding a leading zero (`.1_1` → `0.1_1`).
+    if text.contains('_') {
+        return if text.starts_with('.') { Cow::Owned(format!("0{text}")) } else { text };
+    }
+
     let mut copied_or_ignored_chars = 0usize;
     let mut iter = text.bytes().enumerate();
     let mut curr = iter.next();

crates/oxc_formatter/tests/fixtures/js/literal/numeric-separator.js

@@ -0,0 +1,4 @@
+// Issue #18968 - numeric separators should be preserved
+const x = 123.000_000_000_000_000_000_000_0;
+const y = 1_000_000;
+const z = 0.000_1;

crates/oxc_formatter/tests/fixtures/js/literal/numeric-separator.js.snap

@@ -0,0 +1,27 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// Issue #18968 - numeric separators should be preserved
+const x = 123.000_000_000_000_000_000_000_0;
+const y = 1_000_000;
+const z = 0.000_1;
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// Issue #18968 - numeric separators should be preserved
+const x = 123.000_000_000_000_000_000_000_0;
+const y = 1_000_000;
+const z = 0.000_1;
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// Issue #18968 - numeric separators should be preserved
+const x = 123.000_000_000_000_000_000_000_0;
+const y = 1_000_000;
+const z = 0.000_1;
+
+===================== End =====================