feat(minifier): implement folding substring string fns

crates/oxc_ecmascript/src/lib.rs

@@ -11,6 +11,7 @@ mod string_char_at;
 mod string_char_code_at;
 mod string_index_of;
 mod string_last_index_of;
+mod string_substring;
 mod string_to_big_int;
 mod string_to_number;
 mod to_big_int;
@@ -30,6 +31,7 @@ pub use self::{
     private_bound_identifiers::PrivateBoundIdentifiers, prop_name::PropName,
     string_char_at::StringCharAt, string_char_code_at::StringCharCodeAt,
     string_index_of::StringIndexOf, string_last_index_of::StringLastIndexOf,
-    string_to_big_int::StringToBigInt, string_to_number::StringToNumber, to_big_int::ToBigInt,
-    to_boolean::ToBoolean, to_int_32::ToInt32, to_number::ToNumber, to_string::ToJsString,
+    string_substring::StringSubstring, string_to_big_int::StringToBigInt,
+    string_to_number::StringToNumber, to_big_int::ToBigInt, to_boolean::ToBoolean,
+    to_int_32::ToInt32, to_number::ToNumber, to_string::ToJsString,
 };

crates/oxc_ecmascript/src/string_substring.rs

@@ -0,0 +1,37 @@
+use crate::ToInt32;
+
+pub trait StringSubstring {
+    /// `String.prototype.substring ( start , end ] )`
+    /// <https://tc39.es/ecma262/#sec-string.prototype.substring>
+    fn substring(&self, start: Option<f64>, end: Option<f64>) -> String;
+}
+
+impl StringSubstring for &str {
+    #[expect(clippy::cast_sign_loss)]
+    fn substring(&self, start: Option<f64>, end: Option<f64>) -> String {
+        let start = start.map_or(0, |x| x.to_int_32().max(0) as usize);
+        let end = end.map_or(usize::MAX, |x| x.to_int_32().max(0) as usize);
+        let start = start.min(self.len());
+        let end = end.min(self.len());
+        if start > end {
+            return String::new();
+        }
+        self.chars().skip(start).take(end - start).collect()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_prototype_last_index_of() {
+        use super::StringSubstring;
+        assert_eq!("foo".substring(Some(1.0), None), "oo");
+        assert_eq!("foo".substring(Some(1.0), Some(2.0)), "o");
+        assert_eq!("foo".substring(Some(1.0), Some(1.0)), "");
+        assert_eq!("foo".substring(Some(1.0), Some(0.0)), "");
+        assert_eq!("foo".substring(Some(0.0), Some(0.0)), "");
+        assert_eq!("foo".substring(Some(0.0), Some(1.0)), "f");
+        assert_eq!("abc".substring(Some(0.0), Some(2.0)), "ab");
+        assert_eq!("abcde".substring(Some(0.0), Some(2.0)), "ab");
+    }
+}

crates/oxc_minifier/src/ast_passes/peephole_replace_known_methods.rs

@@ -2,7 +2,7 @@ use cow_utils::CowUtils;
 use oxc_ast::ast::*;
 use oxc_ecmascript::{
     constant_evaluation::ConstantEvaluation, StringCharAt, StringCharCodeAt, StringIndexOf,
-    StringLastIndexOf,
+    StringLastIndexOf, StringSubstring,
 };
 use oxc_traverse::{Traverse, TraverseCtx};
 
@@ -76,7 +76,12 @@ impl PeepholeReplaceKnownMethods {
                     ctx,
                 ),
                 // TODO: Implement the rest of the string methods
-                "substring" | "slice" => None,
+                "substring" | "slice" => Self::try_fold_string_substring_or_slice(
+                    call_expr.span,
+                    call_expr,
+                    string_lit,
+                    ctx,
+                ),
                 "charAt" => {
                     Self::try_fold_string_char_at(call_expr.span, call_expr, string_lit, ctx)
                 }
@@ -131,6 +136,53 @@ impl PeepholeReplaceKnownMethods {
         )));
     }
 
+    fn try_fold_string_substring_or_slice<'a>(
+        span: Span,
+        call_expr: &CallExpression<'a>,
+        string_lit: &StringLiteral<'a>,
+        ctx: &mut TraverseCtx<'a>,
+    ) -> Option<Expression<'a>> {
+        if call_expr.arguments.len() > 2 {
+            return None;
+        }
+
+        let start_idx = if let Some(v) = call_expr.arguments.first() {
+            let val = match v {
+                Argument::SpreadElement(_) => None,
+                _ => Ctx(ctx).get_side_free_number_value(v.to_expression()),
+            }?;
+            Some(val)
+        } else {
+            None
+        };
+        let end_idx = if let Some(v) = call_expr.arguments.get(1) {
+            let val = match v {
+                Argument::SpreadElement(_) => None,
+                _ => Ctx(ctx).get_side_free_number_value(v.to_expression()),
+            }?;
+            Some(val)
+        } else {
+            None
+        };
+
+        #[expect(clippy::cast_precision_loss)]
+        if start_idx.is_some_and(|start| start > string_lit.value.len() as f64 || start < 0.0)
+            || end_idx.is_some_and(|end| end > string_lit.value.len() as f64 || end < 0.0)
+        {
+            return None;
+        }
+
+        if let (Some(start), Some(end)) = (start_idx, end_idx) {
+            if start > end {
+                return None;
+            }
+        };
+
+        return Some(ctx.ast.expression_from_string_literal(
+            ctx.ast.string_literal(span, string_lit.value.as_str().substring(start_idx, end_idx)),
+        ));
+    }
+
     fn try_fold_string_char_at<'a>(
         span: Span,
         call_expr: &CallExpression<'a>,
@@ -393,7 +445,6 @@ mod test {
     }
 
     #[test]
-    #[ignore]
     fn test_fold_string_substring() {
         fold("x = 'abcde'.substring(0,2)", "x = 'ab'");
         fold("x = 'abcde'.substring(1,2)", "x = 'b'");
@@ -412,7 +463,6 @@ mod test {
     }
 
     #[test]
-    #[ignore]
     fn test_fold_string_slice() {
         fold("x = 'abcde'.slice(0,2)", "x = 'ab'");
         fold("x = 'abcde'.slice(1,2)", "x = 'b'");