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Debuggers

There are two debuggers in Jsi. The command-line one:

jsish -d myscr.jsi arg1

and a web GUI one:

jsish -D myscr.jsi arg1

The debugger will stop at "debugger" statements, and anytime an error or warning occurs (ie. in "strict mode").

Commands

The usual debugger commands are supported:

Command Argument Description
break file:line/func Break execution at a named function, line or file
continue Continue execution
delete id Delete one or all breakpoints
disable id Disable one or all breakpoints
down count Move down one or more stack levels
enable id Enable one or all breakpoints
eval expression Evaluate expression in program context
finish Run till return of current function
help string Display command usage
info args/locals/bp/var Show status info
list proc/line count List file lines
next Single-step over function calls
print name Print value of variable
quit Quit debugging current program and exit
step Single-step into function calls
tbreak Set a temporary breakpoint that is disabled when hit
up count Move up one or more stack levels
where Display current location
xeval expression Does eval in debugger context

Example Session

For example:

// Simple test script for the debugger.
function foo2() {
  debugger;
}

function foo1() {
  var x = 99;
  foo2();
}

function foo() {
  var x = 88;
  foo1();
}

foo();
puts("SIMPLE");

a sample debugging session is:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> s
CMD: step
#2:/tmp/simple.js:12    var x = 88; <in function foo()>
#2==>
#2:/tmp/simple.js:13    foo1(); <in function foo()>
#2==>
#3:/tmp/simple.js:7    var x = 99; <in function foo1()>
#3==> c
CMD: continue
#4:/tmp/simple.js:3    debugger; <in function foo2()>
#4==> up
CMD: up
#3:/tmp/simple.js:8    foo2(); <in function foo1()>
#3==> p x
CMD: print
RESULT= "99"
#3==> up
CMD: up
#2:/tmp/simple.js:13    foo1(); <in function foo()>
#2==> p x
CMD: print
RESULT= "88"
#2==>

Note that just like gdb, an empty commands repeats the previous command.

Listing

Source file lines can be displayed with the list command:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> c
CMD: continue
#4:/tmp/simple.js:3    debugger; <in function foo2()>
#2==> l 1 20
CMD: list
FILE: /tmp/simple.js:1
1    : // Simple test script for the debugger.
2    : function foo2() {
3    :   debugger;
4    : }
5    :
6    : function foo1() {
7    :   var x = 99;
8    :   foo2();
9    : }
10   :
11   : function foo() {
12   :   var x = 88;

With no arguments the default is to list 10 lines starting from the current line in the current file. Optional arguments are startLine|func numLines file

Print

The print command can be used to display the value of variables:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> s
CMD: step
#2:/tmp/simple.js:12    var x = 88; <in function foo()>
#2==> s
CMD: step
#2:/tmp/simple.js:13    foo1(); <in function foo()>
#2==> p x
CMD: print
RESULT= "88"

Eval

The eval command can be used to evaluate expresssions or modify values:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> c
CMD: continue
#4:/tmp/simple.js:3    debugger; <in function foo2()>
#4==> up
CMD: up
#3:/tmp/simple.js:8    foo2(); <in function foo1()>
#3==> p x
CMD: print
RESULT= "99"
#3==> ev x=9
CMD: eval
RESULT= 9
#3==> p x
CMD: print
RESULT= "9"

Breakpoints

Breakpoints can be set, cleared and queried as in the following example:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> s
CMD: step
#2:/tmp/simple.js:12    var x = 88; <in function foo()>
#2==> b
CMD: break
breakpoint #1 set: /tmp/simple.js:12
#2==> b foo
CMD: break
breakpoint #2 set: foo
#2==> c
CMD: continue
Stopped at breakpoint #2
#2:/tmp/simple.js:13    foo1(); <in function foo()>
#2==> info
CMD: info
#1    : enabled=true, hits=0, file=/tmp/simple.js:12
#2    : enabled=true, hits=1, func=foo
#2==> dis 1
CMD: disable
#2==> info
CMD: info
#1    : enabled=false, hits=0, file=/tmp/simple.js:12
#2    : enabled=true,  hits=1, func=foo

Note that the debugger will stop whenever a debugger statement is encountered.

Where

The where command outputs a stack backtrace:

jsish -d /tmp/simple.js
#1:/tmp/simple.js:16  foo();
#1==> c
CMD: continue
#4:/tmp/simple.js:3    debugger; <in function foo2()>
#4==> wh
CMD: where
{ fileName:"/tmp/simple.js", funcName:"foo2", level:4, line:3 }
{ fileName:"/tmp/simple.js", funcName:"foo1", level:3, line:8 }
{ fileName:"/tmp/simple.js", funcName:"foo", level:2, line:13 }
{ fileName:"/tmp/simple.js", funcName:"", level:1, line:16 }
#4==> up 2
CMD: up
#2:/tmp/simple.js:13    foo1(); <in function foo()>
#2==> wh
CMD: where
{ fileName:"/tmp/simple.js", funcName:"foo", level:2, line:13 }
{ fileName:"/tmp/simple.js", funcName:"", level:1, line:16 }
#2==>

Implementation

The debugger runs the target script in a sub-interpreter.

The sources are here:

Warning: Jsi bends the memory management rules of Sub-interps a bit to make debugging mode work, which means a small amount of memory may leak during debug sessions.

Debugging Aids

Strict Mode

Strict-mode, which helps find bugs in scripts, is enabled adding use strict to the top of the file:

"use strict";
function foo (a:number, b:string='ok'):number {}

Best to do this, before resorting to a debugger.

See Strict Mode.

Error Diagnostics

Jsi does not generate tracebacks upon error. Instead it provides gcc style warnings that contain the file and line number. For example:

var x = 1;
foo(x)
/home/user/myjsi/foo.js:2: error: 'foo', sub-commands are: Array Boolean Date File
   Function Interp JSON Math Number Object RegExp Sqlite String Websocket alert
   assert clearInterval console decodeURI encodeURI exit file format include info
   isFinite isNaN load parseFloat parseInt puts quote setInterval setTimeout signal
   sys util zvfs.

The file and line number is reported, as well as an enumeration of known commands in the given scope. This allows errors to be parsable by IDE's.

Method Introspection

Upon error, an objects sub-methods are enumerated, if possible:

Info.xx()
Output
error: 'info', sub-commands are: cmds data error events executable funcs
    named platform revisions script vars version.    (at or near string "xx")

Displayed Arguments

And arguments to builtin methods part of the diagnostic:

format();
Output
error: missing args, expected "format(format?,arg,arg?)"