Fix to_js_uint32 not overflowing

crates/swc_ecma_transforms_optimization/src/simplify/expr/tests.rs

@@ -673,6 +673,11 @@ fn test_issue_9256() {
     fold("5e-324 << 5e-324", "0");
     fold("5e-324 << 0", "0");
     fold("0 << 5e-324", "0");
+
+    // Wasn't broken prior, used to ensure overflows are handled correctly
+    fold("1 << 31", "-2147483648");
+    fold("-8 >> 2", "-2");
+    fold("-8 >>> 2", "1073741822");
 }
 
 #[test]

crates/swc_ecma_utils/src/number.rs

@@ -61,7 +61,13 @@ impl ToJsUint32 for f64 {
         const TWO_32: f64 = u32::MAX as f64 + 1.0; // 2^32
 
         let pos_int = self.signum() * self.abs().floor();
-        (pos_int % TWO_32) as u32
+        let result = pos_int % TWO_32;
+        // Extra step: since `x as u32` doesn't overflow, we have to add if result is negative
+        (if result < 0.0 {
+            result + TWO_32
+        } else {
+            result
+        }) as u32
     }
 }
 
@@ -92,6 +98,6 @@ mod tests {
         assert_eq!(f64::MIN.to_js_uint32(), 0);
         assert_eq!(f64::MAX.to_js_uint32(), 0);
 
-        assert_eq!(5.2.to_js_uint32(), 5);
+        assert_eq!((-8.0).to_js_uint32(), 4294967288);
     }
 }