escape(_,0)

SpiderMonkey の escape() は第二引数にビットフラグを受け取る。
Amazon の API 用にこんな関数を書いてて気付いた。

function amazEnc(str)
  encodeURIComponent(str).replace(/[!\'()]+/g, escape).replace(/[*]/g, '%2A');

amazEnc('unescape(_)') //=> Error: invalid string escape mask 8

この 8 は replace() が escape() に渡したマッチ位置。
該当ソースコードを見てみる。

/*
 * Contributions from the String class to the set of methods defined for the
 * global object.  escape and unescape used to be defined in the Mocha library,
 * but as ECMA decided to spec them, they've been moved to the core engine
 * and made ECMA-compliant.  (Incomplete escapes are interpreted as literal
 * characters by unescape.)
 */

/*
 * Stuff to emulate the old libmocha escape, which took a second argument
 * giving the type of escape to perform.  Retained for compatibility, and
 * copied here to avoid reliance on net.h, mkparse.c/NET_EscapeBytes.
 */

#define URL_XALPHAS     ((uint8) 1)
#define URL_XPALPHAS    ((uint8) 2)
#define URL_PATH        ((uint8) 4)

static const uint8 urlCharType[256] =
/*      Bit 0           xalpha          -- the alphas
 *      Bit 1           xpalpha         -- as xalpha but
 *                             converts spaces to plus and plus to %20
 *      Bit 2 ...       path            -- as xalphas but doesn't escape '/'
 */
    /*   0 1 2 3 4 5 6 7 8 9 A B C D E F */
    {    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,       /* 0x */
         0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,       /* 1x */
         0,0,0,0,0,0,0,0,0,0,7,4,0,7,7,4,       /* 2x   !"#$%&'()*+,-./  */
         7,7,7,7,7,7,7,7,7,7,0,0,0,0,0,0,       /* 3x  0123456789:;<=>?  */
         7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,       /* 4x  @ABCDEFGHIJKLMNO  */
         7,7,7,7,7,7,7,7,7,7,7,0,0,0,0,7,       /* 5X  PQRSTUVWXYZ[\]^_  */
         0,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,       /* 6x  `abcdefghijklmno  */
         7,7,7,7,7,7,7,7,7,7,7,0,0,0,0,0,       /* 7X  pqrstuvwxyz{\}~  DEL */
         0, };

/* This matches the ECMA escape set when mask is 7 (default.) */

#define IS_OK(C, mask) (urlCharType[((uint8) (C))] & (mask))

http://mxr.mozilla.org/mozilla-central/source/js/src/jsstr.cpp#304

なにやら歴史的理由も綴られているが，つまりは非エスケープ文字の制限を外せるようにしてあるらしい。

escape('0Aa*+-./')    //=> 0Aa*+-./
escape('0Aa*+-./', 0) //=> %30%41%61%2A%2B%2D%2E%2F

数字アルファベットごと変換でき便利である。
冒頭の関数もシンプルになった。

function amazEnc(str)(
  encodeURIComponent(str).replace(/[!\'()*]+/g, function(s) escape(s, 0)));