fixes & improvements to [].join()

fixes
- [a].join() => "" + a
- ["a", , "b"].join() => "a,,b"
- ["a", null, "b"].join() => "a,,b"
- ["a", undefined, "b"].join() => "a,,b"

improvements
- ["a", "b"].join(null) => "anullb"
- ["a", "b"].join(undefined) => "a,b"
- [a + "b", c].join("") => a + "b" + c

closes #1453

1 files changed, 39 insertions, 23 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 0dcfb2ba..2ba2982e 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2290,39 +2290,57 @@ merge(Compressor.prototype, {
                 }).transform(compressor);
             }
             else if (exp instanceof AST_Dot && exp.expression instanceof AST_Array && exp.property == "join") EXIT: {
-                var separator = self.args.length == 0 ? "," : self.args[0].evaluate(compressor)[1];
-                if (separator == null) break EXIT; // not a constant
-                var elements = exp.expression.elements.reduce(function(a, el){
+                var separator;
+                if (self.args.length > 0) {
+                    separator = self.args[0].evaluate(compressor);
+                    if (separator.length < 2) break EXIT; // not a constant
+                    separator = separator[1];
+                }
+                var elements = [];
+                var consts = [];
+                exp.expression.elements.forEach(function(el) {
                     el = el.evaluate(compressor);
-                    if (a.length == 0 || el.length == 1) {
-                        a.push(el);
+                    if (el.length > 1) {
+                        consts.push(el[1]);
                     } else {
-                        var last = a[a.length - 1];
-                        if (last.length == 2) {
-                            // it's a constant
-                            var val = "" + last[1] + separator + el[1];
-                            a[a.length - 1] = [ make_node_from_constant(compressor, val, last[0]), val ];
-                        } else {
-                            a.push(el);
+                        if (consts.length > 0) {
+                            elements.push(make_node(AST_String, self, {
+                                value: consts.join(separator)
+                            }));
+                            consts.length = 0;
                         }
+                        elements.push(el[0]);
                     }
-                    return a;
-                }, []);
+                });
+                if (consts.length > 0) {
+                    elements.push(make_node(AST_String, self, {
+                        value: consts.join(separator)
+                    }));
+                }
                 if (elements.length == 0) return make_node(AST_String, self, { value: "" });
-                if (elements.length == 1) return elements[0][0];
+                if (elements.length == 1) {
+                    if (elements[0].is_string(compressor)) {
+                        return elements[0];
+                    }
+                    return make_node(AST_Binary, elements[0], {
+                        operator : "+",
+                        left     : make_node(AST_String, self, { value: "" }),
+                        right    : elements[0]
+                    });
+                }
                 if (separator == "") {
                     var first;
-                    if (elements[0][0] instanceof AST_String
-                        || elements[1][0] instanceof AST_String) {
-                        first = elements.shift()[0];
+                    if (elements[0].is_string(compressor)
+                        || elements[1].is_string(compressor)) {
+                        first = elements.shift();
                     } else {
                         first = make_node(AST_String, self, { value: "" });
                     }
                     return elements.reduce(function(prev, el){
-                        return make_node(AST_Binary, el[0], {
+                        return make_node(AST_Binary, el, {
                             operator : "+",
                             left     : prev,
-                            right    : el[0],
+                            right    : el
                         });
                     }, first).transform(compressor);
                 }
@@ -2331,9 +2349,7 @@ merge(Compressor.prototype, {
                 var node = self.clone();
                 node.expression = node.expression.clone();
                 node.expression.expression = node.expression.expression.clone();
-                node.expression.expression.elements = elements.map(function(el){
-                    return el[0];
-                });
+                node.expression.expression.elements = elements;
                 return best_of(self, node);
             }
         }