Use ES6 entries to get more letter.

jsfuck.js

@@ -25,7 +25,7 @@
 
   var MAPPING = {
     'a':   '(false+"")[1]',
-    'b':   '(Function("return{}")()+"")[2]',
+    'b':   '([]["entries"]()+"")[2]',
     'c':   '([]["filter"]+"")[3]',
     'd':   '(undefined+"")[2]',
     'e':   '(true+"")[3]',
@@ -33,7 +33,7 @@
     'g':   '(false+[0]+String)[20]',
     'h':   '(+(101))["to"+String["name"]](21)[1]',
     'i':   '([false]+undefined)[10]',
-    'j':   '(Function("return{}")()+"")[10]',
+    'j':   '([]["entries"]()+"")[3]',
     'k':   '(+(20))["to"+String["name"]](21)',
     'l':   '(false+"")[2]',
     'm':   '(Number+"")[11]',
@@ -101,9 +101,9 @@
     '>':   '("")["italics"]()[2]',
     '?':   '(RegExp()+"")[2]',
     '@':   USE_CHAR_CODE,
-    '[':   '(Function("return{}")()+"")[0]',
+    '[':   '([]["entries"]()+"")[0]',
     '\\':  USE_CHAR_CODE,
-    ']':   '(Function("return{}")()+"")["slice"]("-1")',
+    ']':   '([]["entries"]()+"")[22]',
     '^':   USE_CHAR_CODE,
     '_':   USE_CHAR_CODE,
     '`':   USE_CHAR_CODE,

output.txt

@@ -41,7 +41,7 @@
 `H` 6353
 `I` 70
 `J` 6324
-`K` 7955
+`K` 6486
 `L` 6453
 `M` 4764
 `N` 16
@@ -57,14 +57,14 @@
 `X` 6358
 `Y` 6363
 `Z` 6329
-`[` 1640
+`[` 171
 `\` 6458
-`]` 2043
+`]` 189
 `^` 6339
 `_` 6327
 ``` 6327
 `a` 15
-`b` 1646
+`b` 177
 `c` 144
 `d` 22
 `e` 25
@@ -72,7 +72,7 @@
 `g` 740
 `h` 1714
 `i` 27
-`j` 1648
+`j` 182
 `k` 1703
 `l` 19
 `m` 729