Develop Reference

Comparison with possibly non-existent properties

My background is more in strongly-typed languages than not, so I will state up front that I have a bias against treating undefined objects in a casual manner. That said, I was considering a simple question of assigning a Boolean value to a variable, where said Boolean comes from the property of some object, and while the object will always exist the property may not.
That sounds more complicated than it really is. Take a look. We start with spinning up some objects early in the code, covering the three possible scenarios:
var object1 = {};
object1.IsDisplayed = true;
var object2 = {};
object2.IsDisplayed = false;
var object3 = {};
// notice we do *not* create the IsDisplayed property here
Sometime later we call the process() function that takes a single argument. Let's call this baseline Solution A.
Solution A
This being JavaScript, that actually works as expected for object1, object2, and object3. But it seems to have a bit of a code smell to me; it is not clear that the author has considered a possibly non-existent property.
function process(renderedItem) {
var rendered = renderedItem.IsDisplayed;
if (rendered) {
// do some stuff here
}
}
I see two possible ways to make this more robust:
Solution B
I like this solution because it is both robust and clear to even the most casual reader that the undefined case has been properly considered. On the downside it is wordy, not nearly as compact as solution C, and it uses a string name of a property, which I have a strong distaste for.
function process(renderedItem) {
var rendered = renderedItem.hasOwnProperty('IsDisplayed')
? renderedItem.IsDisplayed
: false;
if (rendered) {
// do some stuff here
}
}
Solution C
I like this solution because it is robust and compact; it explicitly takes into account a possibly non-existent property; however, it is not nearly as obvious as solution B. I remember early on reading some colleagues' code and thinking,"What the heck?" and suggested deleting the === true as redundant. Also, it still is technically referencing an undefined value (in the case of passing in object3).
function process(renderedItem) {
var rendered = renderedItem.IsDisplayed === true;
if (rendered) {
// do some stuff here
}
}
Is there a clear "best practice" amongst these three variations?

Javascript Dynamic object properties through functions returning strings

I'm building a Backbone Marionette Application. In Marionette you can do something like:
Marionette.Application.extend({
regions: {
mainRegion: function(){return someCondition?"#A":"#B"},
otherRegion: "#something"
}
})
I'm trying to implement this in my application with custom objects. What is the best way to achieve this?
Currently, I'm checking for every value:
if(typeof obj == "function") {
this.data.obj = obj();
}
else {
this.data.obj = obj;
}
or the corresponding ?: expression.
Is there an easier way to do this?

Underscore has a result method that does exactly what you want (see http://underscorejs.org/#result)
You can find the implementation here: http://underscorejs.org/docs/underscore.html#section-128

There is a potential solution (cue screaming) in adding value() [not to be confused with valueOf()] to every prototype you're interested in.
Function.prototype.value = function () { return this(); }; // Probably should be .call() so you can do arguments if necessary.
String.prototype.value = function () { return this; };
etc.
Then you can use this.data.obj = obj.value() for all calls.
If you're queasy about adding functions to prototypes, this isn't a solution you'd want to use. Otherwise you could write a wrapper function that takes obj as an argument and returns the right thing based on the type.
The duck typing idea proposed by dandavis is also good IMHO.

How do I add a new method to strings in JavaScript?

Maybe this question have beend posted before what I just do not know how to search for it.
I'd like to know how can I create a method like .replace(), or .toString(). I mean, if I have a variable and I want to search if that variable have number or not, like to do this
var someVariable = "hello 34, how are you";
var containsIntsNumber = someVariable.SearchInteger();//being search integer my custom method
if(containsIntsNumber )
{
console.log("It does have integers");
}
How can I achieve this?

You can modify the prototype on the String object.
String.prototype.someFunction = function () {
/* Your function body here; you can use
this to access the string itself */
};

You can add it to the string prototype.
String.prototype.SearchInteger = function(){
//do stuff
}
the you can call it like this
var someVariable = "hello 34, how are you";
var containsIntsNumber = someVariable.SearchInteger();
Adding additional functions to prototypes can be a bit controversial in the JS community. Be warned that it will then show up when you enumerate over the properties of the variable, and it could theoretically be overwritten or used for a different purpose by an external library.

This can be achieved in few ways. Have a function that return boolean value or extend string prototype so that you can call this method directly on string variable.
This will check wheather string has a number.
String.prototype.hasInteger = function(){
return /\d/.test(this);
}
However it is not recommended to augment native objects, so my suggestion would be just use a function.
function hasInteger(value){
return /\d/.test(value);
}

if(!String.prototype.SearchInteger)
{
Object.defineProperty(String.prototype, 'SearchInteger',
{
value: function()
{
// body of your function here
},
enumerable: false
});
}

You will have to extend the prototype of String in this case. As String is an inbuilt type, It is not recommended to extend their prototype, but you can still do if you fancy(but dont!)
easy example would be something like
String.prototype.SearchInteger = function () {
return this.test(/^.*\d+.*$/g);
};
this should work, though I didn't test.

Comparing function pointers in javascript

Is there any way to comparing function pointers in javascript? Basically, I want to see if I've added the same function multiple times to an array, and then only add it once. Yeah, I can program my way around it, but it would be much easier to do it this way.
The code below does NOT use an array, but illustrates the point I'm trying to make. I'd like for oldPointer to only be set if a myPointer is a different function.
Here is some example code:
function test()
{
}
test.prototype.Loaded = function()
{
this.loaded = true;
}
test.prototype.Add = function(myPointer)
{
if (this.oldPointer != myPointer) //never the same
{
this.oldPointer = myPointer;
}
}
test.prototype.run = function()
{
this.Add(this.Loaded.bind(this));
this.Add(this.Loaded.bind(this)); //this.oldPointer shouldn't be reassigned, but it is
}
var mytest = new test();
test.run();

Assuming bind is a function that uses Function.apply() to create a function closure binding this to the context, this.Loaded.bind(this) will produce a new function every time it is called. That is why your code does not work. Unfortunately there is no way to reference this.Loaded from the function object produced by bind(), so comparison is impossible.
If instead you did something like the below, your check would work, though I'm not sure how much use it would be to you.
test.prototype.run = function()
{
var loadedFn = this.Loaded.bind(this);
this.Add(loadedFn);
this.Add(loadedFn);
}
Please clarify exactly what you are trying to do if you want a better answer.

If your question is "How do I efficiently avoid adding the same function to a given array twice?" the simplest way to program around it is obviously:
// Add f to a if and only if it is not already in a
if (a.indexOf(f) < 0) {
a.push(f);
}
If the linear complexity of indexOf bothers you, and you are only concerned about a single array, you can get very fancy and store the fact that the function was loaded in the function itself:
// Add f to a if and only if it is not already in a
if (! f.alreadyAddedToA) {
a.push(f);
f.alreadyAddedToA = true;
}
Pick any name for the hack property.
If there are multple arrays you are worried about, you can store a kind of hashmap (hacked objects in JS, with suitable keys) inside the function.

Are there legitimate uses for JavaScript's "with" statement?

Alan Storm's comments in response to my answer regarding the with statement got me thinking. I've seldom found a reason to use this particular language feature, and had never given much thought to how it might cause trouble. Now, I'm curious as to how I might make effective use of with, while avoiding its pitfalls.
Where have you found the with statement useful?

Another use occurred to me today, so I searched the web excitedly and found an existing mention of it: Defining Variables inside Block Scope.
Background
JavaScript, in spite of its superficial resemblance to C and C++, does not scope variables to the block they are defined in:
var name = "Joe";
if ( true )
{
var name = "Jack";
}
// name now contains "Jack"
Declaring a closure in a loop is a common task where this can lead to errors:
for (var i=0; i<3; ++i)
{
var num = i;
setTimeout(function() { alert(num); }, 10);
}
Because the for loop does not introduce a new scope, the same num - with a value of 2 - will be shared by all three functions.
A new scope: let and with
With the introduction of the let statement in ES6, it becomes easy to introduce a new scope when necessary to avoid these problems:
// variables introduced in this statement
// are scoped to each iteration of the loop
for (let i=0; i<3; ++i)
{
setTimeout(function() { alert(i); }, 10);
}
Or even:
for (var i=0; i<3; ++i)
{
// variables introduced in this statement
// are scoped to the block containing it.
let num = i;
setTimeout(function() { alert(num); }, 10);
}
Until ES6 is universally available, this use remains limited to the newest browsers and developers willing to use transpilers. However, we can easily simulate this behavior using with:
for (var i=0; i<3; ++i)
{
// object members introduced in this statement
// are scoped to the block following it.
with ({num: i})
{
setTimeout(function() { alert(num); }, 10);
}
}
The loop now works as intended, creating three separate variables with values from 0 to 2. Note that variables declared within the block are not scoped to it, unlike the behavior of blocks in C++ (in C, variables must be declared at the start of a block, so in a way it is similar). This behavior is actually quite similar to a let block syntax introduced in earlier versions of Mozilla browsers, but not widely adopted elsewhere.

I have been using the with statement as a simple form of scoped import. Let's say you have a markup builder of some sort. Rather than writing:
markupbuilder.div(
markupbuilder.p('Hi! I am a paragraph!',
markupbuilder.span('I am a span inside a paragraph')
)
)
You could instead write:
with(markupbuilder){
div(
p('Hi! I am a paragraph!',
span('I am a span inside a paragraph')
)
)
}
For this use case, I am not doing any assignment, so I don't have the ambiguity problem associated with that.

As my previous comments indicated, I don't think you can use with safely no matter how tempting it might be in any given situation. Since the issue isn't directly covered here, I'll repeat it. Consider the following code
user = {};
someFunctionThatDoesStuffToUser(user);
someOtherFunction(user);
with(user){
name = 'Bob';
age = 20;
}
Without carefully investigating those function calls, there's no way to tell what the state of your program will be after this code runs. If user.name was already set, it will now be Bob. If it wasn't set, the global name will be initialized or changed to Bob and the user object will remain without a name property.
Bugs happen. If you use with you will eventually do this and increase the chances your program will fail. Worse, you may encounter working code that sets a global in the with block, either deliberately or through the author not knowing about this quirk of the construct. It's a lot like encountering fall through on a switch, you have no idea if the author intended this and there's no way to know if "fixing" the code will introduce a regression.
Modern programming languages are chocked full of features. Some features, after years of use, are discovered to be bad, and should be avoided. Javascript's with is one of them.

I actually found the with statement to be incredibly useful recently. This technique never really occurred to me until I started my current project - a command line console written in JavaScript. I was trying to emulate the Firebug/WebKit console APIs where special commands can be entered into the console but they don't override any variables in the global scope. I thought of this when trying to overcome a problem I mentioned in the comments to Shog9's excellent answer.
To achieve this effect, I used two with statements to "layer" a scope behind the global scope:
with (consoleCommands) {
with (window) {
eval(expression);
}
}
The great thing about this technique is that, aside from the performance disadvantages, it doesn't suffer the usual fears of the with statement, because we're evaluating in the global scope anyway - there's no danger of variables outside our pseudo-scope from being modified.
I was inspired to post this answer when, to my surprise, I managed to find the same technique used elsewhere - the Chromium source code!
InjectedScript._evaluateOn = function(evalFunction, object, expression) {
InjectedScript._ensureCommandLineAPIInstalled();
// Surround the expression in with statements to inject our command line API so that
// the window object properties still take more precedent than our API functions.
expression = "with (window._inspectorCommandLineAPI) { with (window) { " + expression + " } }";
return evalFunction.call(object, expression);
}
EDIT: Just checked the Firebug source, they chain 4 with statements together for even more layers. Crazy!
const evalScript = "with (__win__.__scope__.vars) { with (__win__.__scope__.api) { with (__win__.__scope__.userVars) { with (__win__) {" +
"try {" +
"__win__.__scope__.callback(eval(__win__.__scope__.expr));" +
"} catch (exc) {" +
"__win__.__scope__.callback(exc, true);" +
"}" +
"}}}}";

Yes, yes and yes. There is a very legitimate use. Watch:
with (document.getElementById("blah").style) {
background = "black";
color = "blue";
border = "1px solid green";
}
Basically any other DOM or CSS hooks are fantastic uses of with. It's not like "CloneNode" will be undefined and go back to the global scope unless you went out of your way and decided to make it possible.
Crockford's speed complaint is that a new context is created by with. Contexts are generally expensive. I agree. But if you just created a div and don't have some framework on hand for setting your css and need to set up 15 or so CSS properties by hand, then creating a context will probably be cheaper then variable creation and 15 dereferences:
var element = document.createElement("div"),
elementStyle = element.style;
elementStyle.fontWeight = "bold";
elementStyle.fontSize = "1.5em";
elementStyle.color = "#55d";
elementStyle.marginLeft = "2px";
etc...

You can define a small helper function to provide the benefits of with without the ambiguity:
var with_ = function (obj, func) { func (obj); };
with_ (object_name_here, function (_)
{
_.a = "foo";
_.b = "bar";
});

Hardly seems worth it since you can do the following:
var o = incrediblyLongObjectNameThatNoOneWouldUse;
o.name = "Bob";
o.age = "50";

I don't ever use with, don't see a reason to, and don't recommend it.
The problem with with is that it prevents numerous lexical optimizations an ECMAScript implementation can perform. Given the rise of fast JIT-based engines, this issue will probably become even more important in the near future.
It might look like with allows for cleaner constructs (when, say, introducing a new scope instead of a common anonymous function wrapper or replacing verbose aliasing), but it's really not worth it. Besides a decreased performance, there's always a danger of assigning to a property of a wrong object (when property is not found on an object in injected scope) and perhaps erroneously introducing global variables. IIRC, latter issue is the one that motivated Crockford to recommend to avoid with.

Visual Basic.NET has a similar With statement. One of the more common ways I use it is to quickly set a number of properties. Instead of:
someObject.Foo = ''
someObject.Bar = ''
someObject.Baz = ''
, I can write:
With someObject
.Foo = ''
.Bar = ''
.Baz = ''
End With
This isn't just a matter of laziness. It also makes for much more readable code. And unlike JavaScript, it does not suffer from ambiguity, as you have to prefix everything affected by the statement with a . (dot). So, the following two are clearly distinct:
With someObject
.Foo = ''
End With
vs.
With someObject
Foo = ''
End With
The former is someObject.Foo; the latter is Foo in the scope outside someObject.
I find that JavaScript's lack of distinction makes it far less useful than Visual Basic's variant, as the risk of ambiguity is too high. Other than that, with is still a powerful idea that can make for better readability.

I think the obvious use is as a shortcut. If you're e.g. initializing an object you simply save typing a lot of "ObjectName." Kind of like lisp's "with-slots" which lets you write
(with-slots (foo bar) objectname
"some code that accesses foo and bar"
which is the same as writing
"some code that accesses (slot-value objectname 'foo) and (slot-value objectname 'bar)""
It's more obvious why this is a shortcut then when your language allows "Objectname.foo" but still.

You can use with to introduce the contents of an object as local variables to a block, like it's being done with this small template engine.

Using "with" can make your code more dry.
Consider the following code:
var photo = document.getElementById('photo');
photo.style.position = 'absolute';
photo.style.left = '10px';
photo.style.top = '10px';
You can dry it to the following:
with(document.getElementById('photo').style) {
position = 'absolute';
left = '10px';
top = '10px';
}
I guess it depends whether you have a preference for legibility or expressiveness.
The first example is more legible and probably recommended for most code. But most code is pretty tame anyway. The second one is a bit more obscure but uses the expressive nature of the language to cut down on code size and superfluous variables.
I imagine people who like Java or C# would choose the first way (object.member) and those who prefer Ruby or Python would choose the latter.

Having experience with Delphi, I would say that using with should be a last-resort size optimization, possibly performed by some kind of javascript minimizer algorithm with access to static code analysis to verify its safety.
The scoping problems you can get into with liberal use of the with statement can be a royal pain in the a** and I wouldn't want anyone to experience a debugging session to figure out what the he.. is going on in your code, only to find out that it captured an object member or the wrong local variable, instead of your global or outer scope variable which you intended.
The VB with statement is better, in that it needs the dots to disambiguate the scoping, but the Delphi with statement is a loaded gun with a hairtrigger, and it looks to me as though the javascript one is similar enough to warrant the same warning.

Using with is not recommended, and is forbidden in ECMAScript 5 strict mode. The recommended alternative is to assign the object whose properties you want to access to a temporary variable.
Source: Mozilla.org

The with statement can be used to decrease the code size or for private class members, example:
// demo class framework
var Class= function(name, o) {
var c=function(){};
if( o.hasOwnProperty("constructor") ) {
c= o.constructor;
}
delete o["constructor"];
delete o["prototype"];
c.prototype= {};
for( var k in o ) c.prototype[k]= o[k];
c.scope= Class.scope;
c.scope.Class= c;
c.Name= name;
return c;
}
Class.newScope= function() {
Class.scope= {};
Class.scope.Scope= Class.scope;
return Class.scope;
}
// create a new class
with( Class.newScope() ) {
window.Foo= Class("Foo",{
test: function() {
alert( Class.Name );
}
});
}
(new Foo()).test();
The with-statement is very usefull if you want to modify the scope, what is necessary for having your own global scope that you can manipulate at runtime. You can put constants on it or certain helper functions often used like e.g. "toUpper", "toLower" or "isNumber", "clipNumber" aso..
About the bad performance I read that often: Scoping a function won't have any impact on the performance, in fact in my FF a scoped function runs faster then an unscoped:
var o={x: 5},r, fnRAW= function(a,b){ return a*b; }, fnScoped, s, e, i;
with( o ) {
fnScoped= function(a,b){ return a*b; };
}
s= Date.now();
r= 0;
for( i=0; i < 1000000; i++ ) {
r+= fnRAW(i,i);
}
e= Date.now();
console.log( (e-s)+"ms" );
s= Date.now();
r= 0;
for( i=0; i < 1000000; i++ ) {
r+= fnScoped(i,i);
}
e= Date.now();
console.log( (e-s)+"ms" );
So in the above mentioned way used the with-statement has no negative effect on performance, but a good one as it deceases the code size, what impacts the memory usage on mobile devices.

Using with also makes your code slower in many implementation, as everything now gets wrapped in an extra scope for lookup. There's no legitimate reason for using with in JavaScript.

I think the with-statement can come in handy when converting a template language into JavaScript. For example JST in base2, but I've seen it more often.
I agree one can program this without the with-statement. But because it doesn't give any problems it is a legitimate use.

It's good for putting code that runs in a relatively complicated environment into a container: I use it to make a local binding for "window" and such to run code meant for a web browser.

I think the object literal use is interesting, like a drop-in replacement for using a closure
for(var i = nodes.length; i--;)
{
// info is namespaced in a closure the click handler can access!
(function(info)
{
nodes[i].onclick = function(){ showStuff(info) };
})(data[i]);
}
or the with statement equivilent of a closure
for(var i = nodes.length; i--;)
{
// info is namespaced in a closure the click handler can access!
with({info: data[i]})
{
nodes[i].onclick = function(){ showStuff(info) };
}
}
I think the real risk is accidently minipulating variables that are not part of the with statement, which is why I like the object literal being passed into with, you can see exactly what it will be in the added context in the code.

I created a "merge" function which eliminates some of this ambiguity with the with statement:
if (typeof Object.merge !== 'function') {
Object.merge = function (o1, o2) { // Function to merge all of the properties from one object into another
for(var i in o2) { o1[i] = o2[i]; }
return o1;
};
}
I can use it similarly to with, but I can know it won't affect any scope which I don't intend for it to affect.
Usage:
var eDiv = document.createElement("div");
var eHeader = Object.merge(eDiv.cloneNode(false), {className: "header", onclick: function(){ alert("Click!"); }});
function NewObj() {
Object.merge(this, {size: 4096, initDate: new Date()});
}

For some short code pieces, I would like to use the trigonometric functions like sin, cos etc. in degree mode instead of in radiant mode. For this purpose, I use an AngularDegreeobject:
AngularDegree = new function() {
this.CONV = Math.PI / 180;
this.sin = function(x) { return Math.sin( x * this.CONV ) };
this.cos = function(x) { return Math.cos( x * this.CONV ) };
this.tan = function(x) { return Math.tan( x * this.CONV ) };
this.asin = function(x) { return Math.asin( x ) / this.CONV };
this.acos = function(x) { return Math.acos( x ) / this.CONV };
this.atan = function(x) { return Math.atan( x ) / this.CONV };
this.atan2 = function(x,y) { return Math.atan2(x,y) / this.CONV };
};
Then I can use the trigonometric functions in degree mode without further language noise in a with block:
function getAzimut(pol,pos) {
...
var d = pos.lon - pol.lon;
with(AngularDegree) {
var z = atan2( sin(d), cos(pol.lat)*tan(pos.lat) - sin(pol.lat)*cos(d) );
return z;
}
}
This means: I use an object as a collection of functions, which I enable in a limited code region for direct access. I find this useful.

I think that the usefulness of with can be dependent on how well your code is written. For example, if you're writing code that appears like this:
var sHeader = object.data.header.toString();
var sContent = object.data.content.toString();
var sFooter = object.data.footer.toString();
then you could argue that with will improve the readability of the code by doing this:
var sHeader = null, sContent = null, sFooter = null;
with(object.data) {
sHeader = header.toString();
sContent = content.toString();
sFooter = content.toString();
}
Conversely, it could be argued that you're violating the Law of Demeter, but, then again, maybe not. I digress =).
Above all else, know that Douglas Crockford recommends not using with. I urge you to check out his blog post regarding with and its alternatives here.

I just really don't see how using the with is any more readable than just typing object.member. I don't think it's any less readable, but I don't think it's any more readable either.
Like lassevk said, I can definitely see how using with would be more error prone than just using the very explicit "object.member" syntax.

You got to see the validation of a form in javascript at W3schools http://www.w3schools.com/js/js_form_validation.asp where the object form is "scanned" through to find an input with name 'email'
But i've modified it to get from ANY form all the fields validate as not empty, regardless of the name or quantity of field in a form. Well i've tested only text-fields.
But the with() made things simpler. Here's the code:
function validate_required(field)
{
with (field)
{
if (value==null||value=="")
{
alert('All fields are mandtory');return false;
}
else
{
return true;
}
}
}
function validate_form(thisform)
{
with (thisform)
{
for(fiie in elements){
if (validate_required(elements[fiie])==false){
elements[fiie].focus();
elements[fiie].style.border='1px solid red';
return false;
} else {elements[fiie].style.border='1px solid #7F9DB9';}
}
}
return false;
}

CoffeeScript's Coco fork has a with keyword, but it simply sets this (also writable as # in CoffeeScript/Coco) to the target object within the block. This removes ambiguity and achieves ES5 strict mode compliance:
with long.object.reference
#a = 'foo'
bar = #b

My
switch(e.type) {
case gapi.drive.realtime.ErrorType.TOKEN_REFRESH_REQUIRED: blah
case gapi.drive.realtime.ErrorType.CLIENT_ERROR: blah
case gapi.drive.realtime.ErrorType.NOT_FOUND: blah
}
boils down to
with(gapi.drive.realtime.ErrorType) {switch(e.type) {
case TOKEN_REFRESH_REQUIRED: blah
case CLIENT_ERROR: blah
case NOT_FOUND: blah
}}
Can you trust so low-quality code? No, we see that it was made absolutely unreadable. This example undeniably proves that there is no need for with-statement, if I am taking readability right ;)

using "with" statement with proxy objects
I recently want to write a plugin for babel that enables macros. I would like to have a separate variable namespace that keeps my macro variables, and I can run my macro codes in that space. Also, I want to detect new variables that are defined in the macro codes(Because they are new macros).
First, I choose the vm module, but I found global variables in the vm module like Array, Object, etc. are different from the main program, and I cant implement module and require that be fully compatible with that global objects(Because I cant reconstruct the core modules). In the end, I find the "with" statement.
const runInContext = function(code, context) {
context.global = context;
const proxyOfContext = new Proxy(context, { has: () => true });
let run = new Function(
"proxyOfContext",
`
with(proxyOfContext){
with(global){
${code}
}
}
`
);
return run(proxyOfContext);
};
This proxy object traps search of all variables and says: "yes, I have that variable." and If the proxy object doesn't really have that variable, show its value as undefined.
In this way, if any variable is defined in the macro code with the var statement, I can find it in the context object(like the vm module). But variables that are defined with let or const only available in that time and will not be saved in the context object(the vm module saves them but doesn't expose them).
Performance: Performance of this method is better than vm.runInContext.
safety: If you want to run code in a sandbox, this is not safe in any way, and you must use the vm module. It only provides a new namespace.

Here's a good use for with: adding new elements to an Object Literal, based on values stored in that Object. Here's an example that I just used today:
I had a set of possible tiles (with openings facing top, bottom, left, or right) that could be used, and I wanted a quick way of adding a list of tiles which would be always placed and locked at the start of the game. I didn't want to keep typing types.tbr for each type in the list, so I just used with.
Tile.types = (function(t,l,b,r) {
function j(a) { return a.join(' '); }
// all possible types
var types = {
br: j( [b,r]),
lbr: j([l,b,r]),
lb: j([l,b] ),
tbr: j([t,b,r]),
tbl: j([t,b,l]),
tlr: j([t,l,r]),
tr: j([t,r] ),
tl: j([t,l] ),
locked: []
};
// store starting (base/locked) tiles in types.locked
with( types ) { locked = [
br, lbr, lbr, lb,
tbr, tbr, lbr, tbl,
tbr, tlr, tbl, tbl,
tr, tlr, tlr, tl
] }
return types;
})("top","left","bottom","right");

As Andy E pointed out in the comments of Shog9's answer, this potentially-unexpected behavior occurs when using with with an object literal:
for (var i = 0; i < 3; i++) {
function toString() {
return 'a';
}
with ({num: i}) {
setTimeout(function() { console.log(num); }, 10);
console.log(toString()); // prints "[object Object]"
}
}
Not that unexpected behavior wasn't already a hallmark of with.
If you really still want to use this technique, at least use an object with a null prototype.
function scope(o) {
var ret = Object.create(null);
if (typeof o !== 'object') return ret;
Object.keys(o).forEach(function (key) {
ret[key] = o[key];
});
return ret;
}
for (var i = 0; i < 3; i++) {
function toString() {
return 'a';
}
with (scope({num: i})) {
setTimeout(function() { console.log(num); }, 10);
console.log(toString()); // prints "a"
}
}
But this will only work in ES5+. Also don't use with.

I am working on a project that will allow users to upload code in order to modify the behavior of parts of the application. In this scenario, I have been using a with clause to keep their code from modifying anything outside of the scope that I want them to mess around with. The (simplified) portion of code I use to do this is:
// this code is only executed once
var localScope = {
build: undefined,
// this is where all of the values I want to hide go; the list is rather long
window: undefined,
console: undefined,
...
};
with(localScope) {
build = function(userCode) {
eval('var builtFunction = function(options) {' + userCode + '}');
return builtFunction;
}
}
var build = localScope.build;
delete localScope.build;
// this is how I use the build method
var userCode = 'return "Hello, World!";';
var userFunction = build(userCode);
This code ensures (somewhat) that the user-defined code neither has access to any globally-scoped objects such as window nor to any of my local variables through a closure.
Just as a word to the wise, I still have to perform static code checks on the user-submitted code to ensure they aren't using other sneaky manners to access global scope. For instance, the following user-defined code grabs direct access to window:
test = function() {
return this.window
};
return test();