feat(minifier): fold string.length / array.length (#8172)

crates/oxc_ecmascript/src/constant_evaluation/mod.rs

@@ -1,7 +1,7 @@
 use std::{borrow::Cow, cmp::Ordering};
 
 use num_bigint::BigInt;
-use num_traits::Zero;
+use num_traits::{ToPrimitive, Zero};
 
 use oxc_ast::ast::*;
 
@@ -193,6 +193,7 @@ pub trait ConstantEvaluation<'a> {
             Expression::StringLiteral(lit) => {
                 Some(ConstantValue::String(Cow::Borrowed(lit.value.as_str())))
             }
+            Expression::StaticMemberExpression(e) => self.eval_static_member_expression(e),
             _ => None,
         }
     }
@@ -458,6 +459,31 @@ pub trait ConstantEvaluation<'a> {
         }
     }
 
+    fn eval_static_member_expression(
+        &self,
+        expr: &StaticMemberExpression<'a>,
+    ) -> Option<ConstantValue<'a>> {
+        match expr.property.name.as_str() {
+            "length" => {
+                if let Some(ConstantValue::String(s)) = self.eval_expression(&expr.object) {
+                    // TODO(perf): no need to actually convert, only need the length
+                    Some(ConstantValue::Number(s.encode_utf16().count().to_f64().unwrap()))
+                } else {
+                    if expr.object.may_have_side_effects() {
+                        return None;
+                    }
+
+                    if let Expression::ArrayExpression(arr) = &expr.object {
+                        Some(ConstantValue::Number(arr.elements.len().to_f64().unwrap()))
+                    } else {
+                        None
+                    }
+                }
+            }
+            _ => None,
+        }
+    }
+
     /// <https://tc39.es/ecma262/#sec-abstract-relational-comparison>
     fn is_less_than(
         &self,

crates/oxc_minifier/src/ast_passes/peephole_fold_constants.rs

@@ -51,7 +51,9 @@ impl<'a> Traverse<'a> for PeepholeFoldConstants {
                     _ => ctx.eval_unary_expression(e).map(|v| ctx.value_to_expr(e.span, v)),
                 }
             }
-            // TODO: return tryFoldGetProp(subtree);
+            Expression::StaticMemberExpression(e) => {
+                Self::try_fold_static_member_expression(e, ctx)
+            }
             Expression::LogicalExpression(e) => Self::try_fold_logical_expression(e, ctx),
             Expression::ChainExpression(e) => Self::try_fold_optional_chain(e, ctx),
             // TODO: tryFoldGetElem
@@ -97,6 +99,16 @@ impl<'a, 'b> PeepholeFoldConstants {
         None
     }
 
+    fn try_fold_static_member_expression(
+        static_member_expr: &mut StaticMemberExpression<'a>,
+        ctx: Ctx<'a, 'b>,
+    ) -> Option<Expression<'a>> {
+        // TODO: tryFoldObjectPropAccess(n, left, name)
+
+        ctx.eval_static_member_expression(static_member_expr)
+            .map(|value| ctx.value_to_expr(static_member_expr.span, value))
+    }
+
     fn try_fold_logical_expression(
         logical_expr: &mut LogicalExpression<'a>,
         ctx: Ctx<'a, 'b>,
@@ -1504,6 +1516,31 @@ mod test {
         test("(+x & 1) & 2", "+x & 0");
     }
 
+    #[test]
+    fn test_fold_array_length() {
+        // Can fold
+        test("x = [].length", "x = 0");
+        test("x = [1,2,3].length", "x = 3");
+        // test("x = [a,b].length", "x = 2");
+
+        // Not handled yet
+        test("x = [,,1].length", "x = 3");
+
+        // Cannot fold
+        test("x = [foo(), 0].length", "x = [foo(),0].length");
+        test_same("x = y.length");
+    }
+
+    #[test]
+    fn test_fold_string_length() {
+        // Can fold basic strings.
+        test("x = ''.length", "x = 0");
+        test("x = '123'.length", "x = 3");
+
+        // Test Unicode escapes are accounted for.
+        test("x = '123\\u01dc'.length", "x = 4");
+    }
+
     #[test]
     fn test_fold_instance_of() {
         // Non object types are never instances of anything.

tasks/minsize/minsize.snap

@@ -13,7 +13,7 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 555.77 kB  | 274.10 kB  | 270.13 kB  | 91.25 kB   | 90.80 kB   | d3.js     
 
-1.01 MB    | 461.06 kB  | 458.89 kB  | 126.91 kB  | 126.71 kB  | bundle.min.js
+1.01 MB    | 461.05 kB  | 458.89 kB  | 126.89 kB  | 126.71 kB  | bundle.min.js
 
 1.25 MB    | 656.99 kB  | 646.76 kB  | 164.18 kB  | 163.73 kB  | three.js  
 
@@ -23,5 +23,5 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 6.69 MB    | 2.34 MB    | 2.31 MB    | 493.51 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.51 MB    | 3.49 MB    | 910.88 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.51 MB    | 3.49 MB    | 910.84 kB  | 915.50 kB  | typescript.js