feat(minifier): try fold typeof (#408)

crates/oxc_hir/src/hir_util.rs

@@ -0,0 +1,77 @@
+use crate::hir::{
+    ArrayExpressionElement, Expression, ObjectProperty, ObjectPropertyKind, PropertyKey,
+    SpreadElement,
+};
+
+/// Code ported from [closure-compiler](https://github.com/google/closure-compiler/blob/f3ce5ed8b630428e311fe9aa2e20d36560d975e2/src/com/google/javascript/jscomp/NodeUtil.java#LL836C6-L836C6)
+/// Returns true if this is a literal value. We define a literal value as any node that evaluates
+/// to the same thing regardless of when or where it is evaluated. So `/xyz/` and `[3, 5]` are
+/// literals, but the name a is not.
+///
+/// Function literals do not meet this definition, because they lexically capture variables. For
+/// example, if you have `function() { return a; }`.
+/// If it is evaluated in a different scope, then it captures a different variable. Even if
+/// the function did not read any captured variables directly, it would still fail this definition,
+/// because it affects the lifecycle of variables in the enclosing scope.
+///
+/// However, a function literal with respect to a particular scope is a literal.
+/// If `include_functions` is true, all function expressions will be treated as literals.
+pub trait IsLiteralValue<'a, 'b> {
+    fn is_literal_value(&self, include_functions: bool) -> bool;
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for Expression<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        match self {
+            Self::FunctionExpression(_) | Self::ArrowExpression(_) => include_functions,
+            Self::ArrayExpression(expr) => {
+                expr.elements.iter().all(|element| element.is_literal_value(include_functions))
+            }
+            Self::ObjectExpression(expr) => {
+                expr.properties.iter().all(|property| property.is_literal_value(include_functions))
+            }
+            _ => self.is_immutable_value(),
+        }
+    }
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for ArrayExpressionElement<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        match self {
+            Self::SpreadElement(element) => element.is_literal_value(include_functions),
+            Self::Expression(expr) => expr.is_literal_value(include_functions),
+            Self::Elision(_) => true,
+        }
+    }
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for SpreadElement<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        self.argument.is_literal_value(include_functions)
+    }
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for ObjectPropertyKind<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        match self {
+            Self::ObjectProperty(method) => method.is_literal_value(include_functions),
+            Self::SpreadProperty(property) => property.is_literal_value(include_functions),
+        }
+    }
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for ObjectProperty<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        self.key.is_literal_value(include_functions)
+            && self.value.is_literal_value(include_functions)
+    }
+}
+
+impl<'a, 'b> IsLiteralValue<'a, 'b> for PropertyKey<'a> {
+    fn is_literal_value(&self, include_functions: bool) -> bool {
+        match self {
+            Self::Identifier(_) | Self::PrivateIdentifier(_) => false,
+            Self::Expression(expr) => expr.is_literal_value(include_functions),
+        }
+    }
+}

crates/oxc_hir/src/lib.rs

@@ -5,6 +5,7 @@ mod serialize;
 
 pub mod hir;
 mod hir_builder;
+pub mod hir_util;
 pub mod precedence;
 mod visit_mut;
 

crates/oxc_minifier/src/compressor/fold.rs

@@ -4,8 +4,9 @@
 
 #[allow(clippy::wildcard_imports)]
 use oxc_hir::hir::*;
-use oxc_span::Span;
-use oxc_syntax::operator::BinaryOperator;
+use oxc_hir::hir_util::IsLiteralValue;
+use oxc_span::{Atom, Span};
+use oxc_syntax::operator::{BinaryOperator, UnaryOperator};
 
 use super::Compressor;
 
@@ -63,6 +64,12 @@ impl<'a> Compressor<'a> {
                 ),
                 _ => None,
             },
+            Expression::UnaryExpression(unary_expr) => match unary_expr.operator {
+                UnaryOperator::Typeof => {
+                    self.try_fold_typeof(unary_expr.span, &unary_expr.argument)
+                }
+                _ => None,
+            },
             _ => None,
         };
         if let Some(folded_expr) = folded_expr {
@@ -136,4 +143,41 @@ impl<'a> Compressor<'a> {
         }
         Tri::Unknown
     }
+
+    /// Folds 'typeof(foo)' if foo is a literal, e.g.
+    /// typeof("bar") --> "string"
+    /// typeof(6) --> "number"
+    fn try_fold_typeof<'b>(
+        &mut self,
+        span: Span,
+        argument: &'b Expression<'a>,
+    ) -> Option<Expression<'a>> {
+        if argument.is_literal_value(true) {
+            let type_name = match argument {
+                Expression::FunctionExpression(_) | Expression::ArrowExpression(_) => {
+                    Some("function")
+                }
+                Expression::StringLiteral(_) | Expression::TemplateLiteral(_) => Some("string"),
+                Expression::NumberLiteral(_) => Some("number"),
+                Expression::BooleanLiteral(_) => Some("boolean"),
+                Expression::NullLiteral(_)
+                | Expression::ObjectExpression(_)
+                | Expression::ArrayExpression(_) => Some("object"),
+                Expression::Identifier(_) if argument.is_undefined() => Some("undefined"),
+                Expression::UnaryExpression(unary_expr)
+                    if unary_expr.operator == UnaryOperator::Void =>
+                {
+                    Some("undefined")
+                }
+                _ => None,
+            };
+
+            if let Some(type_name) = type_name {
+                let string_literal = self.hir.string_literal(span, Atom::from(type_name));
+                return Some(self.hir.literal_string_expression(string_literal));
+            }
+        }
+
+        None
+    }
 }

crates/oxc_minifier/tests/closure/fold_constants.rs

@@ -1,8 +1,27 @@
 //! <https://github.com/google/closure-compiler/blob/master/test/com/google/javascript/jscomp/PeepholeFoldConstantsTest.java>
 
-use crate::test;
+use crate::{test, test_same};
 
 #[test]
 fn undefined_comparison1() {
     test("undefined == undefined", "!0");
 }
+
+#[test]
+fn js_typeof() {
+    test("x = typeof 1", "x='number'");
+    test("x = typeof 'foo'", "x='string'");
+    test("x = typeof true", "x='boolean'");
+    test("x = typeof false", "x='boolean'");
+    test("x = typeof null", "x='object'");
+    test("x = typeof undefined", "x='undefined'");
+    test("x = typeof void 0", "x='undefined'");
+    test("x = typeof []", "x='object'");
+    test("x = typeof [1]", "x='object'");
+    test("x = typeof [1,[]]", "x='object'");
+    test("x = typeof {}", "x='object'");
+    test("x = typeof function() {}", "x='function'");
+
+    test_same("x=typeof [1,[foo()]]");
+    test_same("x=typeof {bathwater:baby()}");
+}

tasks/minsize/minsize.snap

@@ -11,7 +11,7 @@ Original   -> Minified   -> Gzip       Brotli
 
 555.77 kB  -> 274.97 kB  -> 91.38 kB   77.41 kB   d3.js
 
-977.19 kB  -> 456.46 kB  -> 123.82 kB  107.39 kB  bundle.min.js
+977.19 kB  -> 456.45 kB  -> 123.82 kB  107.40 kB  bundle.min.js
 
 1.25 MB    -> 677.77 kB  -> 166.79 kB  135.23 kB  three.js