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...
So my question is, where have you found the with statement useful?
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.
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.
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";
});
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.
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.
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.
Hardly seems worth it since you can do the following:
var o = incrediblyLongObjectNameThatNoOneWouldUse;
o.name = "Bob";
o.age = "50";
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.
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.
Another use occurred to me today, so i searched the web excitedly and found an existing mention of it: Defining Variables inside Block Scope.
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.
let and withWith the introduction of the let statement in JavaScript 1.7, it becomes easy to introduce a new scope when necessary to avoid these problems:
for (var i=0; i<3; ++i)
{
// variables introduced in this statement
// are scoped to the block following it.
let (num = i)
{
setTimeout(function() { alert(num); }, 10);
}
}
But until other browsers implement it, this will remain limited to Mozilla-targeted code. 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 - this is identical to the behavior of let, but 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).
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...
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.
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.
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.
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.
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.
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()});
}
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.
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;
}
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);" +
"}" +
"}}}}";