pass lhs_constants

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Unreleased
 ## 1.1.15 (09 May 2023)
 - update TerserJS to [5.17.3]
+- add lhs_constants (default is true)
 
 ## 1.1.14 (24 February 2023)
 - update TerserJS to [5.16.5]

README.md

@@ -130,6 +130,7 @@ Available options and their defaults are
     :keep_fnames => false,      # Do not drop names in function definitions
     :passes => 1,               # Number of times to run compress. Raising the number of passes will increase compress time, but can produce slightly smaller code.
     :keep_infinity => false,    # Prevent compression of Infinity to 1/0
+    :lhs_constants => true,     # Moves constant values to the left-hand side of binary nodes. `foo == 42 → 42 == foo`
     :side_effects => true,      # Pass false to disable potentially dropping functions marked as "pure" using pure comment annotation. See UglifyJS documentation for details.
     :switches => true,          # de-duplicate and remove unreachable switch branches
   },                            # Apply transformations to code, set to false to skip

lib/terser.rb

@@ -85,6 +85,7 @@ class Error < StandardError; end
       :keep_fnames => false, # Do not drop names in function definitions
       :passes => 1, # Number of times to run compress. Raising the number of passes will increase compress time, but can produce slightly smaller code.
       :keep_infinity => false, # Prevent compression of Infinity to 1/0
+      :lhs_constants => true, # Moves constant values to the left-hand side of binary nodes. `foo == 42 → 42 == foo`
       :side_effects => true, # Pass false to disable potentially dropping functions marked as "pure" using pure comment annotation. See TerserJS documentation for details.
       :switches => true # de-duplicate and remove unreachable switch branches
     }, # Apply transformations to code, set to false to skip

spec/terser_spec.rb

@@ -702,6 +702,37 @@ class Bar {}
     end
   end
 
+  describe 'lhs_constants' do
+    let(:code) do
+      <<-JS
+        function fun(a) {
+          var x;
+          x = a == 42;
+          x = a === 42;
+          return x;
+        }
+      JS
+    end
+
+    it 'compresses with lhs_constants' do
+      compiled = Terser.compile(code, :compress => {
+                                  :evaluate => true,
+                                  :lhs_constants => true
+                                })
+      expect(compiled).to include("42==n")
+      expect(compiled).to include("42===n")
+    end
+
+    it 'can be disabled to preserve the order' do
+      compiled = Terser.compile(code, :compress => {
+                                  :evaluate => true,
+                                  :lhs_constants => false
+                                })
+      expect(compiled).to include("n==42")
+      expect(compiled).to include("n===42")
+    end
+  end
+
   describe 'quote style' do
     let(:code) do
       <<-JS