Per #385, adding 'null_this' option to rewrite 'null' this-param in call/apply/bind to '0'

README.md

@@ -198,6 +198,10 @@ to set `true`; it's effectively a shortcut for `foo=true`).
 
 - `unsafe` (default: false) -- apply "unsafe" transformations (discussion below)
 
+- `null_this` -- if `null` appears in the `this`-parameter position of `.call(..)`, `.apply(..)`, or `.bind(..)`, rewrite `null` to `0`.
+
+    **Note:** potentially unsafe operation, only use if `null` implies the `this` binding is not relevant for the function call in question.
+
 - `conditionals` -- apply optimizations for `if`-s and conditional
   expressions
 

lib/compress.js

@@ -54,6 +54,7 @@ function Compressor(options, false_by_default) {
         drop_debugger : !false_by_default,
         unsafe        : false,
         unsafe_comps  : false,
+        null_this     : false,
         conditionals  : !false_by_default,
         comparisons   : !false_by_default,
         evaluate      : !false_by_default,
@@ -1782,6 +1783,11 @@ merge(Compressor.prototype, {
                 return make_node(AST_Undefined, self).transform(compressor);
             }
         }
+
+        // check for a `this` param to `call/apply/bind` that's `null`
+        // and rewrite to `0`, if "null_this" option is enabled
+        self.null_this(compressor);
+
         return self.evaluate(compressor)[0];
     });
 
@@ -2301,4 +2307,17 @@ merge(Compressor.prototype, {
     OPT(AST_Object, literals_in_boolean_context);
     OPT(AST_RegExp, literals_in_boolean_context);
 
+    AST_Call.DEFMETHOD("null_this",function(compressor){
+        if (compressor.option("null_this")
+            && this.expression.property
+            && /^(?:call|apply|bind)$/.test(this.expression.property)
+            && this.args.length > 0
+            && this.args[0].start.type === "atom"
+            && this.args[0].end.value === "null") {
+            this.args[0] = make_node(AST_Number, this.args[0], {
+                value: 0
+            });
+        }
+    });
+
 })();

test/compress/null-this.js

@@ -0,0 +1,46 @@
+keep_null_this: {
+    options = {
+    };
+    input: {
+        foo(null,42);
+        call(null,42);
+        apply(null,42);
+        bind(null,42);
+        foo.call(null,42);
+        foo.apply(null,42);
+        foo.bind(null,42);
+    }
+    expect: {
+        foo(null,42);
+        call(null,42);
+        apply(null,42);
+        bind(null,42);
+        foo.call(null,42);
+        foo.apply(null,42);
+        foo.bind(null,42);
+    }
+}
+
+drop_null_this: {
+    options = {
+        null_this: true
+    };
+    input: {
+        foo(null,42);
+        call(null,42);
+        apply(null,42);
+        bind(null,42);
+        foo.call(null,42);
+        foo.apply(null,42);
+        foo.bind(null,42);
+    }
+    expect: {
+        foo(null,42);
+        call(null,42);
+        apply(null,42);
+        bind(null,42);
+        foo.call(0,42);
+        foo.apply(0,42);
+        foo.bind(0,42);
+    }
+}