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see.js: See and debug local variables.

The see.js debugger can be used to inspect and change local variable state when you are not stopped at a breakpoint. This is a good solution for debugging code wrapped in a top-level anonymous closure or for debugging more complicated uses of closures. The see.js debugger also provides simple in-page logging with tree view inspection of objects; and it provides support for debugging using CoffeeScript.


Any local scope can be debugged by calling eval( within the scope of interest. For example, the following nicely encapsulated code would normally be painful to debug without the added see line:

(function() {
  var private_var = 0;
  function myclosuremaker() {
    eval(;  // Debug variables visible in this scope.
    var counter = 0;
    return function() { ++counter; }
  var inc = myclosuremaker();

When eval( is called, the see debugger is shown at the bottom of the page, an en eval hook is set up within the current scope.

The debugging panel works like the Firebug or Chrome debugger console, except that it uses the eval hook to give visibility into local variable scope. In this example, "counter" and "private_var" and "inc" and "myclosuremaker" will all be visible symbols that can be used and manipulated in the debugger.

Debugging multiple scopes

It is possible to attach to multiple scopes with a single program by adding the following at the scope of interest:


To switch scopes within the interactive panel, just enter ":" followed by the scope name, for example, ":scopename". ":top" goes to global scope, and ":" goes back to the default scope defined at init.

Screenshot of see panel


This URL can be used as a bookmarklet that loads see.js on any page.


When you are using the bookmarklet, eval( calls may not be present in the code, but it is possible to insert the see eval loop by evaluating using your regular (Chrome or Firebug) debugger to run eval( when at a breakpoint in the scope of interest.


The see.js script originally started as a teaching tool in a CoffeeScript environment, so it also supports use of CoffeeScript as the console language. Here it how to initialize see.js to interpret code entered in the panel as CoffeeScript instead of Javascript:



The top-level see function logs output to the see panel. Logged objects are shown in a tree view of the object state at the moment when the object is logged.

see(a, b, c);

The panel can be cleared with see.clear().

Using the regular debugger

To inspect an object visible to see in the regular debugger, just use see.eval('mylocal'), which evaluates the expression in the scope of interest and returns the value. To focus on a different named scope, use the two-argument form, see.eval('scopename', 'myexpression').

More examples of usage

see.init();               // Creates the interactive panel with global scope.
see.init({height: 30, title: 'test panel'});   // Sets some options.
eval(see.init());         // Does the same thing as eval(
eval(see.scope('name'));  // Type ":name" in the panel to use this scope.
see(a, b, c);             // Logs values into the panel.
see.loghtml('<b>ok</b>'); // Logs HTML without escaping.
r = see.repr(a, 3);       // Builds a tree representation of a to depth 3.
x = see.noconflict();     // Relinguishes use of the 'see' name; use 'x'.

Options to pass to init

evalThe default function (or closure) to use to evaluate expressions.
thisThe object to use as "this" within the evaluation.
depthThe depth to which to traverse logged and evaluated objects.
heightThe pixel height of the interactive panel.
titleA title shown at the top of the panel.
panelfalse if no interactive panel is desired.
consoleSet to window.console to echo logging to the console also.
historySet to false to disable localStorage use for interactive history.
linestyleCSS style for a single log line.
element(if panel is false) - The element into which to logging is done.
autoscroll(if panel is false) - The element to autoscroll to bottom.
jQueryA local copy of jQuery to reuse instead of loading one.
coffeeThe CoffeeScript compiler object, for coffeescript support.
abbreviateArray of return values (e.g. undefined) to silence in interaction.
noconflictName to use instead of "see".

Implementation notes

If eval(see.init()) or eval(see.scope('name')) is called multiple times for the same name, the scope is reset to the last scope set. If multiple closures are created at the same line of code, that means that you will only see the last one. You can generate a name using eval(see.scope('name' + index)) if you want to preserve visibility into many scopes.

When see.init() is called, a private (noconflict) copy of jQuery is loaded if window.jQuery is not already present on the page (unless the 'panel' option is false, or the 'jQuery' option is explicity supplied to init).

Every expression entered in the panel is stored to '_loghistory' in localStorage unless the 'history' option set to a different key name, or set to false to disable history persistence.