Read this article in: Russian, Chinese, French.
Introduction
Always in programs we declare functions and variables which then successfully use building our systems. But how and where the interpreter finds our data (functions, variable)? What occurs, when we reference to needed objects?
Many ECMAScript programmers know that variables are closely related with the execution context:
var a = 10; // variable of the global context
(function () {
var b = 20; // local variable of the function context
})();
alert(a); // 10
alert(b); // "b" is not defined
Also, many programmers know that the isolated scope in the current version of specification is created only by execution contexts with “function” code type. I.e., in contrast with C/C++, for example the block of for loop in ECMAScript does not create a local context:
for (var k in {a: 1, b: 2}) {
alert(k);
}
alert(k); // variable "k" still in scope even the loop is finished
Let’s see in more details what occurs when we declare our data.
Data declaration
If variables are related with the execution context, it should know where its data are stored and how to get them. This mechanism is called a variable object.
- variables (
var, VariableDeclaration); - function declarations (FunctionDeclaration, in abbreviated form FD);
- and function formal parameters
declared in the context.
Notice, in ES5 the concept of variable object is replaced with lexical environments model, which detailed description can be found in the appropriate chapter.
Schematically and for examples, it is possible to present variable object as a normal ECMAScript object:
VO = {};
And as we said, VO is a property of an execution context:
activeExecutionContext = {
VO: {
// context data (var, FD, function arguments)
}
};
Indirect referencing to variables (via property names of VO) allows only variable object of the global context (where the global object is itself the variable object). For other contexts directly to reference the VO is not possible, it is purely mechanism of implementation.
When we declare a variable or a function, there is nothing else as creation of the new property of the VO with the name and value of our variable.
Example:
var a = 10;
function test(x) {
var b = 20;
};
test(30);
And corresponding variable objects are:
// Variable object of the global context
VO(globalContext) = {
a: 10,
test: <reference to function>
};
// Variable object of the "test" function context
VO(test functionContext) = {
x: 30,
b: 20
};
But at implementation level (and specification) the variable object is an abstract essence. Physically, in concrete execution contexts, VO is named differently and has different initial structure.
Variable object in different execution contexts
Some operations (e.g. variable instantiation) and behavior of the variable object are common for all execution context types. From this viewpoint it is convenient to present the variable object as an abstract base thing. Function context can also define additional details related with the variable object.
AbstractVO (generic behavior of the variable instantiation process)
║
╠══> GlobalContextVO
║ (VO === this === global)
║
╚══> FunctionContextVO
(VO === AO, <arguments> object and <formal parameters> are added)
Let’s consider it in detail.
Variable object in global context
Here, first it is necessary to give definition of the Global object.
Global object is the object which is created before entering any execution context; this object exists in the single copy, its properties are accessible from any place of the program, the life cycle of the global object ends with program end.
At creation the global object is initialized with such properties as Math, String, Date, parseInt etc., and also by additional objects among which can be the reference to the global object itself — for example, in BOM, window property of the global object refers to global object (however, not in all implementations):
global = {
Math: <...>,
String: <...>
...
...
window: global
};
When referencing to properties of global object the prefix is usually omitted, because global object is not accessible directly by name. However, to get access to it is possible via this value in the global context, and also through recursive references to itself, for example window in BOM, therefore write simply:
String(10); // means global.String(10); // with prefixes window.a = 10; // === global.window.a = 10 === global.a = 10; this.b = 20; // global.b = 20;
So, coming back to variable object of the global context — here variable object is the global object itself:
VO(globalContext) === global;
It is necessary to understand accurately this fact since for this reason declaring a variable in the global context, we have ability to reference it indirectly via property of the global object (for example when the variable name is unknown in advance):
var a = new String('test');
alert(a); // directly, is found in VO(globalContext): "test"
alert(window['a']); // indirectly via global === VO(globalContext): "test"
alert(a === this.a); // true
var aKey = 'a';
alert(window[aKey]); // indirectly, with dynamic property name: "test"
Variable object in function context
Regarding the execution context of functions — there VO is inaccessible directly, and its role plays so-called an activation object (in abbreviated form — AO).
VO(functionContext) === AO;
An activation object is created on entering the context of a function and initialized by property arguments which value is the Arguments object:
AO = {
arguments: <ArgO>
};
Arguments object is a property of the activation object. It contains the following properties:
- callee — the reference to the current function;
- length — quantity of real passed arguments;
- properties-indexes (integer, converted to string) which values are the values of function’s arguments (from left to right in the list of arguments). Quantity of these properties-indexes == arguments.length. Values of properties-indexes of the arguments object and present (really passed) formal parameters are shared.
Example:
function foo(x, y, z) {
// quantity of defined function arguments (x, y, z)
alert(foo.length); // 3
// quantity of really passed arguments (only x, y)
alert(arguments.length); // 2
// reference of a function to itself
alert(arguments.callee === foo); // true
// parameters sharing
alert(x === arguments[0]); // true
alert(x); // 10
arguments[0] = 20;
alert(x); // 20
x = 30;
alert(arguments[0]); // 30
// however, for not passed argument z,
// related index-property of the arguments
// object is not shared
z = 40;
alert(arguments[2]); // undefined
arguments[2] = 50;
alert(z); // 40
}
foo(10, 20);
Concerning the last case, in older versions of Google Chrome there was a bug — there parameter z and arguments[2] were also shared.
In ES5 the concept of activation object is also replaced with common and single model of lexical environments.
Phases of processing the context code
Now we have reached the main point of this article. Processing of the execution context code is divided on two basic stages:
- Entering the execution context;
- Code execution.
Modifications of the variable object are closely related with these two phases.
Notice, that processing of these two stages are the general behavior and independent from the type of the context (i.e. it is fair for both: global and function contexts).
Entering the execution context
On entering the execution context (but before the code execution), VO is filled with the following properties (they have already been described at the beginning):
- for each formal parameter of a function (if we are in function execution context)
- for each function declaration (FunctionDeclaration, FD)
- for each variable declaration (var, VariableDeclaration)
— a property of the variable object with a name and value of formal parameter is created; for not passed parameters — property of VO with a name of formal parameter and value undefined is created;
— a property of the variable object with a name and value of a function-object is created; if the variable object already contains a property with the same name, replace its value and attributes;
— a property of the variable object with a variable name and value undefined is created; if the variable name is the same as a name of already declared formal parameter or a function, the variable declaration does not disturb the existing property.
Let’s see on the example:
function test(a, b) {
var c = 10;
function d() {}
var e = function _e() {};
(function x() {});
}
test(10); // call
On entering the test function context with the passed parameter 10, AO is the following:
AO(test) = {
a: 10,
b: undefined,
c: undefined,
d: <reference to FunctionDeclaration "d">
e: undefined
};
Notice, that AO does not contain function x. This is because x is not a function declaration but the function-expression (FunctionExpression, in abbreviated form FE) which does not affect on VO.
However, function _e is also a function-expression, but as we will see below, because of assigning it to the variable e, it becomes accessible via the e name. The difference of a FunctionDeclaration from the FunctionExpression is in detail discussed in Chapter 5. Functions.
And after that there comes the second phase of processing of a context code — the code execution stage.
Code execution
By this moment, AO/VO is already filled by properties (though, not all of them have the real values passed by us, most of them yet still have initial value undefined).
Considering all the same example, AO/VO during the code interpretation is modified as follows:
AO['c'] = 10; AO['e'] = <reference to FunctionExpression "_e">;
Once again I notice that function expression _e is still in memory only because it is saved to declared variable e. But the function expression x is not in the AO/VO. If we try to call x function before or even after definition, we get an error: "x" is not defined. Unsaved to a variable function expression can be called only with its definition (in place) or recursively.
One more (classical) example:
alert(x); // function
var x = 10;
alert(x); // 10
x = 20;
function x() {}
alert(x); // 20
Why in the first alert x is a function and moreover is accessible before the declaration? Why it’s not 10 or 20? Because, according to the rule — VO is filled with function declarations on entering the context. Also, at the same phase, on entering the context, there is a variable declaration x, but as we mentioned above, the step of variable declarations semantically goes after function and formal parameters declarations and on this phase do not disturb the value of the already declared function or formal parameter with the same name. Therefore, on entering the context VO is filled as follows:
VO = {};
VO['x'] = <reference to FunctionDeclaration "x">
// found var x = 10;
// if function "x" would not be already defined
// then "x" be undefined, but in our case
// variable declaration does not disturb
// the value of the function with the same name
VO['x'] = <the value is not disturbed, still function>
And then at code execution phase, VO is modified as follows:
VO['x'] = 10; VO['x'] = 20;
what we can see in the second and third alerts.
In the example below we see again that variables are put into the VO on entering the context phase (so, the else block is never executed, but nevertheless, the variable b exists in VO):
if (true) {
var a = 1;
} else {
var b = 2;
}
alert(a); // 1
alert(b); // undefined, but not "b is not defined"
About variables
Often various articles and even books on JavaScript claim that: “it is possible to declare global variables using var keyword (in the global context) and without using var keyword (in any place)”. It is not so. Remember:
variables are declared only with using var keyword.
And assignments like:
a = 10;
just create the new property (but not the variable) of the global object. “Not the variable” is not in the sense that it cannot be changed, but “not the variable” in concept of variables in ECMAScript (which then also become properties of the global object because of VO(globalContext) === global, we remember, yeah?).
And the difference is the following (let’s show on the example):
alert(a); // undefined alert(b); // "b" is not defined b = 10; var a = 20;
All again depends on VO and phases of its modifications (entering the context stage and the code execution stage):
Entering the context:
VO = {
a: undefined
};
We see that at this phase there is no any b since it is not a variable, b will appear only at code execution phase (but in our case won’t since there is an error).
Let’s change the code:
alert(a); // undefined, we know why b = 10; alert(b); // 10, created at code execution var a = 20; alert(a); // 20, modified at code execution
There is one more important point concerning variables. Variables, in contrast with simple properties, have attribute {DontDelete}, meaning impossibility to remove a variable via the delete operator:
a = 10; alert(window.a); // 10 alert(delete a); // true alert(window.a); // undefined var b = 20; alert(window.b); // 20 alert(delete b); // false alert(window.b); // still 20
Note, in ES5 {DontDelete} is renamed into the [[Configurable]] and can be manually managed via Object.defineProperty method.
However there is one execution context on which this rule does not affect. It is the eval context: there {DontDelete} attribute is not set for variables:
eval('var a = 10;');
alert(window.a); // 10
alert(delete a); // true
alert(window.a); // undefined
For those who test these examples in console of some debug tool, e.g. Firebug: notice, that Firebug also uses eval to execute your code from the console. So there vars also do not have {DontDelete} and can be deleted.
Feature of implementations: property __parent__
As it was already noted, by the standard, to get direct access to the activation object is impossible. However, in some implementations, namely in SpiderMonkey and Rhino, functions have special property __parent__, which is the reference to the activation object (or the global variable object) in which these functions have been created.
Example (SpiderMonkey, Rhino):
var global = this;
var a = 10;
function foo() {}
alert(foo.__parent__); // global
var VO = foo.__parent__;
alert(VO.a); // 10
alert(VO === global); // true
In the example above we see that function foo is created in the global context and, accordingly, its __parent__ property is set to variable object of the global context, i.e. to the global object.
However, to get the activation object in SpiderMonkey with the same way is not possible: depending on the version, __parent__ for inner function returns either null or global object.
In Rhino, access to the activation object is allowed and available via the same way:
Example (Rhino):
var global = this;
var x = 10;
(function foo() {
var y = 20;
// the activation object of the "foo" context
var AO = (function () {}).__parent__;
print(AO.y); // 20
// __parent__ of the current activation
// object is already the global object,
// i.e. the special chain of variable objects is formed,
// so-called, a scope chain
print(AO.__parent__ === global); // true
print(AO.__parent__.x); // 10
})();
Conclusion
In this article we have moved further forward in studying of objects related with execution contexts. I hope the material is useful and has clarified some aspects and ambiguities which, probably, you had before. Further by the plan, the next chapters will be devoted to the Scope chain, Identifier resolution and, as consequence, Closures.
If you have questions, feel free to ask them in comments.
Additional literature
- 10.1.3 – Variable Instantiation;
- 10.1.5 – Global Object;
- 10.1.6 – Activation Object;
- 10.1.8 – Arguments Object.
Translated by: Dmitry A. Soshnikov.
Published on: 2010-03-15
Originally written by: Dmitry A. Soshnikov [ru, read »]
Originally published on: 2009-06-27
Nice writeup. By VO you mean the Reference I assume?
@qFox
Thanks; about VO — this is a just my abbreviation for “Variable object”; for shortness, not to repeat every time “variable object” phrase as there are to many in this text. Also, it is convenient to use in formulas and talks (e.g. saying that current scope chain is a VO|AO + [[Scope]] — which means, that variable/activation object is added in front of parent scope chain).
About “Reference” — what exactly do you mean? Yes, it is a reference to object, but VO|AO — is a purely implementation mechanism; only VO of global context is accessible directly and as implementation feature — AO in Rhino. Or maybe you meant Reference type? If so, VO|AO can be used as a base property of Reference type value.
Dmitry.
excelent articles If you have in mind to translate in spanish maybe I can help
thanks for the info
@Juan Anzaldo
Thanks, Juan. Yes, it is possible to translate it to Spanish (I’m glad to see that there are many Spanish guys interested deeply in ECMAScript). But unfortunately I don’t know Spanish at all. So if you take care about the English => Spanish translation, I’ll help with detailed explanations (or with clarifying of ambiguities) if will be needed.
Dmitry.
So that’s why
vars should go at the beginning of scripts!These articles are awesome.
@Alejandro Moreno
Yep, sometimes it is convenient to define all
vars at the beginning, because anyway they all are parsed on entering the context.But for some
vars it is also useful to declare them at the code position where they are used (e.g. loop counters) — just because it’s convenient and you don’t want to see all auxiliary helper variables at the beginning if e.g. you exit the code in the first line if some check isn’t passed. Although one should keep in mind that variable still will be available after the loop is finished.Dmitry.
Good article,I have learned a lot form here. So I translate it into Chines:
ECMA-262-3 深入解析.第二章.变量对象
thank you!
@denisdeng
Thanks, that’s great. Added a link to your translation.
Thank you! I will translate the next chapters as soon as possible。
I have a question:
“properties-indexes (integer, converted to string) “,
I think you want to say:
“properties-indexes [integer, converted to string]”
it’s difference at “()” or “[]”
Do you think my understanding correct?
@justin
Oh, nope, in that sentence was mentioned not a code expression, but just simple grammar punctuation. I notice there what exactly is an “index-property” (or “array-index” which is described in 15.4 of ES3 spec). Because ES3 has only properties and these properties can be only strings.
I could change it to array indexes definition (because ES3 has such concept), but then arguments object could be treated as an array instance, but it is not (although, I think that it is an error of the spec and arguments should be an array).
And “()” — was just a wording grammar punctuation there.
Dmitry.
I understand, thank you!
Another problem：
“It is convenient to present variable object as base abstract essence from which concrete sub essences playing role of variable object in different execution contexts are “inherited””
This passage is not easy to understand, not easy to translate.
Here is my translate:
Chinese:
变量对象作为基本抽象事物来表现是很方便的，具体子事物是抽象事物的具体实现，具体子事物在不同的执行上下文中扮演着变量对象的角色，它们是“可继承” 的；
English:
Variable objects to represent things as the basic abstraction is very convenient;
Specific sub-things are abstract and concrete realization of things;
Specific sub-things in a different execution context play the role of the variables object.
For example：
AO is VO’s concrete realization in function execution context;
AO === Specific sub-things
VO === the basic abstraction things
Do you think my understanding correct?
AO inherite form VO?
@justin
Yep, that exactly I meant. Yes, “inherited” word was a little confusing, so I removed it. Added an example illustration to make it easier to represent.
“Inherit” meant mostly that there is some common behavior (i.e. variable instantiation process) which is independent from the execution context type, but at the same time different execution context types can define their own initial structure of VO (e.g. arguments object and formal parameters of a function) and moreover, use different object as storage for variable instantiation: in global context — it is global object itself, in function context — it is an activation object.
Dmitry.
VO/AO,I understand that, thank you very muck!
one more question：）
” but“not the variable” in concept of variables in ECMAScript“
This passage is not easy to understand，I think your meant is :
because it does not meet the concept of variable in ECMAScript standard, so it “is not a variable”
Correct?
@justing
Yep, correct. The main goal that variables in ECMAScript (1) cannot be deleted (except the eval context) with using of delete operator and (2) instantiated on entering the context. Exactly these two facts were meant in sentence “is a variable in concept of variables in ECMAScript”.
Meanwhile, unqualified identifiers of the global object (i.e. simple assignment to non-existent yet identifier, such as: a = 10) — not: they can be deleted and created at code execution stage.
Dmitry.
Thanks so much, through this article I learned a lot!
After disscussing with you,I think I have understood this article totally , and also translated it into Chinese .
I’m sure my translation is more nearly to this excellent article.
here it is : http://www.cnblogs.com/justinw/archive/2010/04/23/1718733.html
@justin
OK, I will add the second version of Chinese translation for this chapter. Also you can discuss it with Denis (which also provides translations) about this question. Because I, unfortunately, can’t say which Chinese translation is better (because don’t know Chinese at all) and automatic Google-translate also cannot provide such info for me.
Dmitry.
I understand, thanks!
I have a suggestion
If the article is updated, it is best to maintain an update log
then facilitate future updates Other languages
@justin
In some other articles I would make something like “updated on <date>“. What actually I did in comments of e.g. The Quiz article when was changing some questions (because it was important for those who already passed the test with previous questions).
But this series is academical so I don’t want to break article descriptions with some update, update, update words. It should be complete and up-to-date information without breaking the text itself.
But for own purposes (and to keep translations also up-to-date) — yeah, I’ll think about it and maybe will make sort of subversion repository for that.
Dmitry.
Hi Dmitry,
I have tested this following bit of code in the firebug console, in Google Chrome, and Firefox. The code works as expected in Chrome, but does not in firefox. This is because, executed in Firefox, the global object “window” is not equals to the context object “this”. Could you explain why ?
Thanks,
Serge.
————————————-
var a = new String('test'); alert(a); // directly, is found in VO(globalContext): "test" alert(window['a']); // indirectly via global === VO(globalContext): "test" alert(a === this.a); // true var aKey = 'a'; alert(window[aKey]); // indirectly, with dynamic property name: "test"@sergeM
Yes, exactly. Firebug evaluates its console’s code in several wrapper functions and applying
withstatement (where the argument ofwithstatement is the _FirebugCommandLine object).You may check this by evaluating in Firebug’s console the following code:
In contrast, Chrome’s console evaluates code in context of the global object, i.e.
thisvalue === global object.Though in versions of Firebug it’s should be fixed and
thisshould be set to global object.Dmitry.
Hello Dmitry, Thanks for your answer. By the way, your articles on ecmascript are an excellent and unique source to understand the intrinsics and subtilities of this language. Please, keep up the good work.
Serge.
Hello Dmitry,I’ve got a question here.As you mentioned in this article’s index:
Seems you didn’t explain what the variable object is like in a eval execution context.And in your another article Javascript.The Core,you said :
Does that mean unlike global EC and function EC,a eval EC use its caller’s VO instead of creating its own VO?I’m confused and I will really appreciate it if you could give some more details on it.
Thanks,
Anderson.
@Anderson
Yes, exactly. It is mentioned in the first chapter devoted to execution contexts. The same situation with the scope chain —
evalborrows it from the calling context (i.e. context of the caller).Although, maybe I’ll add a small note to this chapter too (reminding it).
Notice however, that it relates to ES3 and non-strict mode of the ES5. In the strict mode of the ES5,
evalcannot create a variable/function in the caller’s context, since it evaluates in its sandbox environment.Dmitry.
Hi Dmitry. Wonderful series of articles.
The entire ECMAScript Language Specification ELS 262 Fifth edition is available, online, in linkable HTML form, at http://www.ecma262-5.com.
Didn’t know if you were aware of this.
-rm
@Richard Mallonee
Hi Richard,
Yeah, I’m aware about this online version. I already use it in the ES5 series (in the section of additional literature). Your work is also very useful, thanks for sharing it online.
However, it would be good to have there also links to all published erratas.
Dmitry.
Dmitry,
What reasoning do you attribute to the variables declared within if-else, switch, for, do-while?
I mean what is causing the variables declared within the above statements to attain global scope?
Should a novice developer (like me) assume variables declared within control structures are like any other variables declared in global scope?
thanks
-nag
@nag
In some languages there is a block-scope variables. Such variables are being removed after the scope of block ends.
For instance, in C:
// global int foo = 10; if (true) { // local int foo = 20; printf("%d", foo); // 20 } // block ends, its scope too, including local "foo" // again global printf("%d", foo); // 10In JavaScript (in current version of standard) there is no block-scope local variables; only — function-scope local variables.
That means:
// global var foo = 10; if (true) { var foo = 20; alert(foo); // 20 } // block ends, but "foo" wasn't local alert(foo); // 20After careful reading of this chapter, you should know, that all variables are created on entering the context stage. This means, that second
varin the JS example above is ignored, because there is already a “foo” var (notice, that if the local var in if-statement block was named e.g. “bar”, it also would created on entering the context). And only then, at code execution stage, there is just an assignment in the if-block to global “foo”.P.S.: actually, JS has some case of block-scope var, however, it’s not an explicit
var, but the argument of thecatch-clause:var foo = 10;
try { throw 20; } catch (foo) { alert(foo); // 20 } // again global alert(foo); // 10Dmitry.
Dimtry,
I think catch() must be a type of function.
I tried eval(), which treats local variables as belonging to global scope.
Thank you very much for explaining in such a great detail.
-nag
Dmitry,
Excerpt from your last comment::
P.S.: actually, JS has some case of block-scope var, however, it’s not an explicit var, but the argument of the catch-clause:
var foo = 10;
try {
throw 20;
} catch (foo) {
alert(foo); // 20
}
// again global
alert(foo); // 10
The scenario elucidated above holds good in firefox.
IE treats foo as global.
Thus the result of final alert(foo); statement is 20.
-nag
@nag
No, it’s a clause of the
trystatement, it’s not a function. However, yeah,catchadds a special object, containing its argument (in this case — “foo”) in the front of current scope chain, shadowing a global “foo” Therefore, we can refer it duringcatchblock is running. After it ends, its special object is removed from the scope chain and again the global “foo” is available. You can find this case described in detail in the fours chapter, devoted a scope chain.evalworks with the calling context and uses the variable object and the scope chain exactly of the calling context (i.e. the context, from which eval is called). Therefore, if a variable is created withineval, it’s created in the calling context; and if you callevalfrom the global context, of course it creates a variable in the global context. This case is described in the first chapter.Notice however, that in the strict mode of ES5,
evalexecutes its code in the sandbox environment and cannot create a variable in the calling context.IE’s bug.
Dmitry.
thank you very much for such a detail elaboration.
-nag
Excellent article, I knew about variable and function hoisting but was unaware of their order within the VO. Great to read a series of articles that get down to the metal of the language, which helps to clarify some of the more intricate features.
Looks like a typo error to me. Shouldn’t it be “impossibility to remove variable”?
@Luiz
Well, variables are also properties of something. This “something” is the variable/activation object. Attribute
{DontDelete}relates exactly to a property, it’s the attribute of the property. So, it’s not a typo, since, repeat, a variable is also a property but with (initially) with its special attributes. However, to avoid confusion, yes, fixed it to “variable”; thanks.Dmitry.
Dmitry, thanks for the great articles!
You wrote:
In my view these variable-property discussions brings the most confusion among the developers.
I would replace them with following:
1. All we need is Data
2. Data is accessed in two ways via Identifiers and via Properties (both of them of Reference Type you are talking about)
3. JavaScript creates an identifier for each variable declaration, function declaration and function formal parameter. The identifiers are stored as properties in Variable Object. The identifiers could not be deleted.
4. JavaScript creates a property for each property declaration. Two ways of doing it:
-> p = 5, no var in front of it
-> var obj = {p: 5}
Properties does not affect the VO (except global properties, because VO === GO in Global Context). Properties could be deleted.
To summarize: The less we use the term variable, the less is the confusion.
I am curious, do you think such explanation is correct?
May be a bit over-simplified considering EcmaScript specification but still…..
//Plamen
@Plamen Todorov
Yes, it’s a good explanation. After all, all of them are just properties of something (either of a VO/AO, or an environment in ES5, or a simple object).
Though, for simplicity a term variable in casual talks (when it’s not required to differentiate some deep details) of course can also be used.
Dmitry.
hi Dmitry, thanks 4 u great article
just one thing i didn’t understand
why a=10 create a property for global object? how?
hi,Dmitry,you’ve shared a serial of posts for me.But I wonder that when a VO/AO destroy ?
Hi,Dmitry!
first of all, your article is really amazing, Keep Rocking! But, in this post, the paragraph “About variables”,you wrote as below:
“variables are declared only with using var keyword.
And assignments like:
a = 10;
just create the new property (but not the variable) of the global object.”
The last sentence, to me, is not very clear. Your expression give me a feeling that in any case, useing assignment statement without
var, should create a property of global.I think, a assignment statement without
varshould trigger a identifier looking-up follow the active context’s scope chain. When got the same property name in AO|VO, change that value, if got nothing, then create the new property of global with the name and value as the each side of=.all I said as the code at below:
function foo(){ var x=20; x=100; y=100; alert(x); //100 alert(window.x); //undefined alert(y); //100 alert(y===window.y); //true } foo();when
foofunction got execution, thevo(foo)had thexproperty, andvo(foo).x=20. The sentencex=100;just revised thevo(foo).xfrom20to100, did not create the new property for global. Buty, the new property of global was created.@piglite, yes, absolutely correct — the second use-case is meant here (when there is no yet such a variable).
Before the assignment, the reference (on the left-hand side) should be resolved. And this is done at the identifiers resolution process through the scope chain.
Notice also, that in strict mode such assignment already fails and doesn’t create a global property.
Excellent!!! I strongly wish to translate your articles into Chinese. Could you allow me to do it?
It is significant to the web developers in china because your great articles illustrate the truth of the concept in great details. It will be very helpful to the web developers in my country.
Thanks in advance!
@360weboy
Thanks, glad it’s useful. Sure, send me the link when you’ll have the articles translated. Notice though, there are already Chinese translations of some chapters of the series.
@Dmitry
Hi,
Okay. As soon as i have done it, i will send you the link. I noticed there is someone translate your articles. But, same great article may be translated differently depending on the understanding of the translator.
Thanks a lot.
Hi, Dmitry! First of all I would like to thank you very much for all that you have written there. It really improved my knowledge on ECMAScript and I think this is a valuable knowledge.
But I’m writing it because there is still one thing that I don’t understand in ES – it’s the exact difference between ES3’s activation objects and ES5’s declarative environment records. I used to distinguish them by thinking that AOs are just normal, plain ECMAScript objects (what suggested that to me was that in the ES3 spec there is written that ‘arguments’ property of AO has the {DontEnum} attribute – exactly just like a regular object) but it was never said that AO is really a plain object. And, since you wrote that environment records are to REPLACE AOs I assume that there is some difference in those concepts. But I can’t find the differences. So: do they differ? If so, in what way? Curiosity is really killing me, even though ES3 is quite an old thing. Thanks in advance.
@Kuba,
The difference is mostly abstract. In ES5 spec, AO/VO are not distinguished in naming anymore, and combine into the concept of environments.
How the environments are physically implemented is up to an implementation (i.e. it still can be a plain JS object). In practice though, engines likely use more optimized storage for declarative lexical environments, with e.g. faster access to variables, etc. Thus, it doesn’t cancel the fact, that the arguments is still
{DonEnum}(or[[Enumerable]] == falsein ES5 terms).So the changes to environments were in order to: (1) unify the concepts, and (2) allow implementations to make optimizations at implementing them.
@Dmitry
Much thansk for your response. You said, however, that environments were introduced to “allow implementations to make optimizations at implementing them.”. If I correctly understand that engines could implement AO/VO as whatever they wanted to (not necessarily as plain objects), and environments are also implemented that way, what omptimizations, that were unable to be done before, can be done now?
@Kuba
Correct. The optimization is one of the reasons. Another (and also important reason) is to abstract away the low-level concept of activation records (that are called as activation objects in ES), and instead to deal with highly-abstracted environments — without making a commitment how these environments are implemented (i.e. it’s still can be a plain JS object in some non-efficient, but fully spec-conformant, implementation, or vice-versa, a highly optimized low-level structure in a sophisticated compiler).
As for whether such optimizations could be done before in ES3 — yes, they could actually (and were/are in practice) — as long as the implementation reflects to the user-level code what spec says. Even if it’s said that AO is a JS object, implementation could choose not to do this, if it’s not observable to the user-code.
More detailed info can be found in the ES5.Ch3.1 Lexical Environments.