Thanks very much for this.  I have been testing it for several days with a legacy application and I think I will be able to make good use of it.

However there are two issues I have found, so far, that make it unusable as it is.

The first is in relation to ESC sequences.  The ESC sequences for Erase in Line and Erase in Display do not handle the case ESC[0J and ESC[0K which should be treated as ESC[J and ESC[K respectively.  I think I know how to download from GIT and add these functions to esc.js source and push a change request but it may be simpler for you to add them.

The second issue is that our legacy application makes extensive use of %X364 mnemonics calls in write commands

e.g. write /CUP(1,1),/ED(0),"This text should appear on a cleared screen at row 1, column 1"

With WebTerminal version 4 as downloaded any use of %X364 mnemonics results in error <NOLINE>WebTerminal.Core.1. 

However I have been able to fix this by appending the code from ^%X364.int into the method redirects of class WebTerminal.Core as below.

You may want to consider adding this to a new release as it makes a whole new set of legacy apps usable with WebTerminal.

Many thanks for all your efforts on this.  If I can work out how I will raise the two problems above in the Github issue tracker.  In the meantime I have patched the version on my development machine and will continue with testing.

/// Cache WEB Terminal version 4.1.2 core.
/// The core class which handles client requests and executes COS code.
/// All writes used here are used for $X and $Y compatibility, but they actually do not
/// write any code to the screen.
Class WebTerminal.Core Extends Common [ Not ProcedureBlock ]
{

/// Write and read redirects used when redirecting i/o.
/// Each of the redirects signals to $ZPARENT process the $LISTBUILD string.
/// There is several actions defined in the WebTerminal.Engine handler class for received list.
/// "o" is for output. Resulting with $lb("o", {string})
/// "r" is for reading string. Resulting with $lb("r", {length}, {timeout})
/// "rc" is for reading char. Resulting with $lb("rc", {timeout})
/// "end" symbolizes that execution end is reached. Resulting with $lb("end", {error message})
Method redirects() [ Private, ProcedureBlock = 0 ]
{
wstr(str)
    do ##class(%Device).ReDirectIO($$$NO)
    write str
    do ##class(%Device).ReDirectIO($$$YES)
    quit ##class(WebTerminal.Common).SendChunk($ZPARENT, "o", str)

wchr(c)
    do ##class(%Device).ReDirectIO($$$NO)
    write $CHAR(c)
    do ##class(%Device).ReDirectIO($$$YES)
    quit ##class(WebTerminal.Common).SendChunk($ZPARENT, "o", $CHAR(c))

wnl
    do ##class(%Device).ReDirectIO($$$NO)
    write !
    do ##class(%Device).ReDirectIO($$$YES)
    quit ##class(WebTerminal.Common).SendChunk($ZPARENT, "o", $CHAR(13, 10))

wff
    do ##class(%Device).ReDirectIO($$$NO)
    set $X = 0
    set $Y = 0
    do ##class(%Device).ReDirectIO($$$YES)
    quit ##class(WebTerminal.Common).SendChunk($ZPARENT, "o", $CHAR(12))

wtab(s)
    do ##class(%Device).ReDirectIO($$$NO)
    set $x = s
    do ##class(%Device).ReDirectIO($$$YES)
    quit ##class(WebTerminal.Common).SendChunk($ZPARENT, "o", $CHAR(27) _ "[" _ (s + 1) _ "G")

rstr(length = 32656, timeout = 86400)
    do ##class(WebTerminal.Common).SendChunk($ZPARENT, "r", $lb(length, timeout))
    quit $LISTGET(##class(WebTerminal.Common).ReceiveChunk(), 2)

rchr(timeout = 86400)
    do ##class(WebTerminal.Common).SendChunk($ZPARENT, "c", timeout)
    quit $LISTGET(##class(WebTerminal.Common).ReceiveChunk(), 2)
 ;
 ;  Added code below to be able to simulate %X364 Mnemonic space
 ;  functionality (e.g.  write /ED(n),EL(n),/SGA(n),etc ) when 
 ;  executing legacy routines in WebTerminal.
 ;
X364FIX
 ; FIXED ERROR - MISSING % - IN DAQ FUNCTION
 ; BINDING FOR ANSI X3.64 NAMESPACE, NOV/92 ; LRS952 06/07/05
 /*
 +--------------------------------------------------------+
 | Copyright 1986-2011 by InterSystems Corporation,       |
 | Cambridge, Massachusetts, U.S.A.                       |
 | All rights reserved.                                   |
 |                                                        |
 | Confidential, unpublished property of InterSystems.    |
 |                                                        |
 | This media contains an authorized copy or copies       |
 | of material copyrighted by InterSystems and is the     |
 | confidential, unpublished property of InterSystems.    |
 | This copyright notice and any other copyright notices  |
 | included in machine readable copies must be reproduced |
 | on all authorized copies.                              |
 +--------------------------------------------------------+
 */
 u 0 w !,"Illegal entry point: do not call ^",$ZN," directly."
 q:$zu(41)'>2  ZTRAP "EP"
erdx s $x=0 q
erdy s $y=0 q
APC ; Application program command
 w $c(27)_"_"
 q
BEL ; Ring the bell
 w $c(7)
 q
CBT(%1) ; Cursor backward tabulation %1 tab stops
 s %1=+$g(%1,1) w $c(27,91)_%1_"Z" s $zt="erdx",$x=$x+7\8-%1*8
 q
CCH ; Cancel character
 w $c(27)_"T"
 q
CHA(%1) ; Cursor horizontal absolute (move to column %1)
 s %1=+$g(%1,1) w $c(27,91)_%1_"G" s $zt="erdx",$x=%1-1
 q
CHT(%1) ; Cursor horizontal tabulation (forward %1 tab stops)
 s %1=+$g(%1,1) w $c(27,91)_%1_"I" s $zt="erdx",$x=$x\8+%1*8
 q
CNL(%1) ; Cursor next line (cursor down %1 lines)
 s %1=+$g(%1,1) w $c(27,91)_%1_"E" s $zt="erdy",$y=$y+%1
 q
CPL(%1) ; Cursor preceding line (cursor up %1 lines)
 s %1=+$g(%1,1) w $c(27,91)_%1_"F" s $zt="erdy",$y=$y-%1
 q
CPR ; Cursor position report (return in $KEY)
 n %1 w $c(27,91)_"6n" r %1
 q
CTC(%1,%2,%3,%4,%5,%6,%7,%8,%9) ; Cursor tabulation control
 n %i,%p s %p=""
 f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_%i)
 w $c(27,91)_%p_"W"
 q
CUB(%1) ; Cursor backward %1 columns
 s %1=+$g(%1,1) w $c(27,91)_%1_"D" s $zt="erdx",$x=$x-%1
 q
CUD(%1) ; Cursor down %1 lines
 s %1=+$g(%1,1) w $c(27,91)_%1_"B" s $zt="erdy",$y=$y+%1
 q
CUF(%1) ; Cursor forward %1 columns
 s %1=+$g(%1,1) w $c(27,91)_%1_"C" s $zt="erdx",$x=$x+%1
 q
CUP(%2,%1) ; Cursor position (column %1, line %2)
 s %1=+$g(%1,1),%2=+$g(%2,1) w $c(27,91)_%2_";"_%1_"H"
 s $zt="ecup",$x=%1-1,$zt="erdy",$y=%2-1 q
ecup s $x=0,$zt="erdy",$y=%2-1
 q
CUU(%1) ; Cursor up %1 lines
 s %1=+$g(%1,1) w $c(27,91)_%1_"A" s $zt="erdy",$y=$y-%1
 q
CVT(%1) ; Cursor vertical tabulation
 w $c(27,91)_+$g(%1,1)_"Y"
 q
DA ; Device attributes - return in $KEY
 n %1,%2,%3 s %3="" u $i:("":"+S")
 w $c(27,91)_"c" r %1 s %2=$k f  r *%1 s %3=%3_$c(%1) q:%1=99
 s $k=%2_%3 u $i:("":"-S")
 q
DAQ(%1,%2,%3,%4,%5,%6,%7,%8,%9) ; Define area qualification
 n %i,%p s %p=""
 //   Original %X364 routine fails on undefined variable i
 // f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_i)
 f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_%i)
 w $c(27,91)_%p_"o"
 q
DCH(%1) ; Delete %1 characters
 w $c(27,91)_+$g(%1,1)_"P"
 q
DCS ; Device control string
 w $c(27)_"P"
 q
DL(%1) ; Delete %1 lines
 w $c(27,91)_+$g(%1,1)_"M"
 q
DMI ; Disable manual input
 q
DSR(%1) ; Device status report - type %1 - return in $KEY
 n %2 w $c(27,91)_+$g(%1,5)_"n" r %2
 q
EA(%1) ; Erase in area
 w $c(27,91)_+$g(%1,1)_"O"
 q
ECH(%1) ; Erase %1 characters
 w $c(27,91)_+$g(%1,1)_"X"
 q
ED(%1) ; Erase in display (%1=0 cursor-to-end,1 begin-to-cursor,2 entire scr)
 w $c(27,91)_+$g(%1)_"J"
 q
EF(%1) ; Erase in field
 w $c(27,91)_+$g(%1,1)_"N"
 q
EL(%1) ; Erase in line (%1=0 cursor-to-end, 1 begin-to-cursor, 2 entire line)
 w $c(27,91)_+$g(%1)_"K"
 q
EMI ; Enable manual input
 q
EPA ; End of protected area
 w $c(27)_"W"
 q
ESA ; End of selected area
 w $c(27)_"G"
 q
FNT ; Font selection
 q
GSM ; Graphic size modification
 q
GSS ; Graphic size selection
 q
HPA(%1) ; Horizontal position attribute (cursor to column %1)
 s %1=+$g(%1,1) w $c(27,91)_%1_"`" s $zt="erdx",$x=%1-1
 q
HPR(%1) ; Horizontal position relative (cursor forward %1 columns)
 s %1=+$g(%1,1) w $c(27,91)_%1_"a" s $zt="erdx",$x=$x+%1
 q
HTJ ; Horizontal tab with justify
 w $c(27)_"I"
 q
HTS ; Horizontal tabulation set
 w $c(27)_"H"
 q
HVP(%1,%2) ; Horizontal and vertical position (column %1, line %2)
 s %1=+$g(%1,1),%2=+$g(%2,1) w $c(27,91)_%2_";"_%1_"f"
 s $zt="ehvp",$x=%1-1,$zt="erdy",$y=%2-1 q
ehvp s $x=0,$zt="erdy",$y=%2-1
 q
ICH(%1) ; Insert %1 characters
 w $c(27,91)_+$g(%1,1)_"@"
 q
IL(%1) ; Insert %1 lines
 w $c(27,91)_+$g(%1,1),"L"
 q
IND ; Index
 w $c(27)_"D" s $y=$y+1
 q
INT ; Interrupt
 w $c(27,91)_"a"
 q
JFY ; Justify
 q
MC ; Media copy
 w $c(27,91)_"i"
 q
MW ; Message waiting
 w $c(27)_"U"
 q
NEL ; Next line
 w $c(27)_"E" s $x=0,$y=$y+1
 q
NP(%1) ; Next page (advance %1 pages of terminal display memory)
 w $c(27,91)_+$g(%1,1)_"U"
 q
OSC ; Operating system command
 w $c(27)_"]"
 q
PLD ; Partial line down
 w $c(27)_"K"
 q
PLU ; Partial line up
 w $c(27)_"L"
 q
PM ; Privacy message
 w $c(27)_"^"
 q
PP(%1) ; Preceding page (backup %1 pages of terminal display memory)
 w $c(27,91)_+$g(%1,1)_"V"
 q
PU1 ; Private use one
 w $c(27)_"Q"
 q
PU2 ; Private use two
 w $c(27)_"R"
 q
QUAD ; QUAD
 q
REP ; Repeat
 w $c(27,91)_"b"
 q
RI ; Reverse index
 w $c(27)_"M" s $zt="erdy",$y=$y-1
 q
RIS ; Reset to initial state
 w $c(27)_"c" s $x=0,$y=0
 q
RM(%1,%2,%3,%4,%5,%6,%7,%8,%9) ; Reset mode
 n %i,%p s %p=""
 f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_%i)
 w $c(27,91)_%p_"l"
 q
SEM ; Select editing extent mode
 w $c(27,91)_"Q"
 q
SGR(%1,%2,%3,%4,%5,%6,%7,%8,%9)  ; Select graphic rendition %1 thru %9
 n %i,%p s %p=""
 f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_%i)
 w $c(27,91)_%p_"m"
 q
SL ; Scroll left
 q
SM(%1,%2,%3,%4,%5,%6,%7,%8,%9) ; Set mode
 n %i,%p s %p=""
 f %i=1:1:9 i $d(@("%"_%i)) s %p=%p_$e(";",%p'="")_@("%"_%i)
 w $c(27,91)_%p_"h"
 q
SPA ; Start of protected area
 w $c(27)_"V"
 q
SPI ; Spacing increment
 q
SR ; Scroll right
 q
SS2 ; Single shift two
 w $c(27)_"N"
 q
SS3 ; Single shift three
 w $c(27)_"O"
 q
SSA ; Start of selected area
 w $c(27)_"F"
 q
ST ; String terminator
 w $c(27)_"\"
 q
STS ; Set transmit state
 w $c(27)_"S"
 q
SU ; Scroll up
 w $c(27,91)_"S"
 q
TBC ; Tabulation clear
 w $c(27,91)_"g"
 q
TSS ; Thin space specification
 q
VPA(%1) ; Vertical position attribute (move to row %1 at same column)
 s %1=+$g(%1,1) w $c(27,91)_%1_"d" s $zt="erdy",$y=%1-1
 q
VPR(%1) ; Vertical position relative (move down %1 lines at same column)
 s %1=+$g(%1,1) w $c(27,91)_%1_"e" s $zt="erdy",$y=$y+%1
 q
VTS ; Vertical tabulation sets
 w $c(27)_"J"
 q
}

ClassMethod VarList() As %String [ ProcedureBlock = 1 ]
{
    if $data(%)
    new % set %=$select($test:$LISTBUILD("%"),1:"")
    set:$data(%0) %=%_$LISTBUILD("%0")
    new %0 set %0="%0"
    for {
        set %0=$ORDER(@%0)
        quit:%0=""
        set %=%_$LISTBUILD(%0, $IsObject(@%0), @%0)
    }
    return %
}

/// Retrieves a command text from the parent process.
/// Terminates itself if the parent process is dead.
ClassMethod WaitCommand() As %String [ ProcedureBlock = 1 ]
{
    for {
        set data = ..ReceiveChunk()
        set flag = $LISTGET(data, 1)
        if (flag = "m") { // message
            return $LISTGET(data, 2)
        } elseif (flag = "a") { // autocomplete
            do ##class(WebTerminal.Common).SendChunk($ZPARENT, "a", ..VarList())
        } else { // end or unexpected
            do $system.Process.Terminate($JOB, 2)
            return ""
        }
    }
}

/// Starts new terminal loop. Must be called with JOB command.
ClassMethod Loop(StartupRoutine As %String = "") As %Status
{
    if ($ZPARENT = 0) {
        write "This method is for JOB use only."
        return 0
    }
    open "terminal"::"^%X364"
    use $io::"^" _ $ZName
    if (StartupRoutine '= "") {
        do $System.Util.SetInterruptEnable($$$YES)
        do ##class(%Device).ReDirectIO($$$YES)
        try {
            do @StartupRoutine
            do ##class(%Device).ReDirectIO($$$NO)
        } catch {
            do ##class(%Device).ReDirectIO($$$NO)
            do ..SendChunk($ZPARENT, "e", $LISTBUILD($NAMESPACE, $ZERROR))
        }
        do ..SendChunk($ZPARENT, "e", $LISTBUILD($NAMESPACE, ""))
        return $$$OK
    }
    kill // kill any temporary variables ProcedureBlock may have
    for {
        do ##class(%Device).ReDirectIO($$$YES)
        do $System.Util.SetInterruptEnable($$$YES)
        set $ZERROR = ""
        try {
            xecute ..WaitCommand()
        } catch {}
        do ##class(%Device).ReDirectIO($$$NO)
        write !! // assume that prompt takes 2 lines
        do ..SendChunk($ZPARENT, "e", $LISTBUILD($NAMESPACE, $ZERROR))
    }
    return $$$OK
}

}