This might be a silly question but most of the frameworks start with
(function(){
//all data
})()
Immediately invoked expression
So that There will be no collision with other js files right,so basically this is a function execution context that gets popped of the stack after execution,
but how come it still could handle asynchronous requests like a click after page has been loaded
For example:
(function(){
//all data
var a=true;
btn.addEventListener("click",()=>{
alert(a);
});
})() ;
The whole code runs first and an event Listener is given to a button but after,that execution context is popped off the stack so how come my button can still contact and get data from the "finished safecode function"?
Is this an implementation of closures where execution stack gets popped off but still the variables needed for dependant nested functions getting hung up?
Yeah, that's just the behavior of closures: a function always has access to everything declared in its outer blocks (lexical scope), even if the outer blocks have already finished execution. Until those functions don't exist anymore, the variables will continue to exist.
Inner functions keep a reference of their parents execution context
Related
Hey everyone i got a question about how Javascript works in memory
i've heard that every execution context has a space in memory for that execution context
so if that so that's mean that all the variables on the program doesn't sits randomly in memory but rather sits in a memory block of the execution context where they created
now i will try to explain it visually
let's say we have a program and inside that program we have a function called createVars
function createVars(){
var name = 'John',lastName = 'Doe',age = 24;
}
when we invoke the createVars function it will create a new execution context and that execution context will have it's own block in memory where it now reserved for that execution context variables and the this keyword and the outer reference
so here is an example let's say we have an unoccupied place in memory
and now we invoke createVars()
does it look like that in memory or the variables are randomly placed ?
I'm trying to understand how the arrow function passed to setTimeout, remembers the value of this from the previous execution context. I know that the this value is looked up using lexical scoping rules when executing arrow functions. Does that mean the arrow function closes over variables and the this keyword?
var obj = {
name: 'TestName',
func: function() {
console.log(this)
setTimeout(() => console.log(this), 1000)
}
}
obj.func() //{name: "TestName", func: ƒ}
Actually the whole point of using the fat arrow notation is to inherit the parent scope. Main application of arrow functions is to pull down the parent scope.
When we invoke a setTimeout the chain happens as follows-:
Call goes into Call-Stack.
It is transferred to Web-Browser api space, which waits till the interval time.
Now after interval time is completed it'll be handled by Event-Loop which will transfer it to JS-queue.
The command execution will wait till other items in queue finish executing and then it'll transfer the execution flow again to Call-Stack whenever it is empty.
Now the command inside setTimeout will be executed.
JS execution flow
Does that mean the arrow function closes over the this keyword?
Yes, that's exactly what happens.
My title pretty much sums it all.
Can anyone enlighten me on...
"What is the 'Execution Context' in JavaScript?"
and on how it relates to 'this,' hoisting, prototype chaining, scope and garbage collection?
You're asking about several different concepts that aren't very closely related. I'll try to briefly address each.
Execution context is a concept in the language spec that—in layman's terms—roughly equates to the 'environment' a function executes in; that is, variable scope (and the scope chain, variables in closures from outer scopes), function arguments, and the value of the this object.
The call stack is a collection of execution contexts.
See also this answer and this article.
Scope is literally that: the scope in which a variable can be accessed. Simplistically:
var x;
function a() {
var y;
}
x can be accessed from anywhere. When a is invoked, x will be in the outer scope. (Stored in the scope chain.)
In contrast, y can only be accessed by code in a() because it is limited to a's scope. This is what the var keyword does: restricts a variable to the local scope. If we omitted var, y would end up in the global scope, generally considered a bad thing.
Think of hoisting as more of a compile-time thing. In JavaScript, function declarations are "hoisted" to the top of their scope. In other words, they are parsed and evaluated before any other code. (This is opposed to function expressions, which are evaluated inline.) Consider the following:
a();
b();
function a() { }
var b = function() { }
The call to a() will succeed because its declaration was hoisted to the top; a was assigned to automatically before program execution began. The call to b() will fail with a TypeError because b will not be defined until line 4.
You have asked so many concepts but lets pick one by one & understand them.
The environment in which your code is running is Execution context.
It is created when your code is executed.
Execution Context (Global), created by JS Engine contains 3 important things for you:
Global object - window
Special Object this
Ref to outer environment
Lets see a simple example to understand Global Execution Context:
var a = "Hello World";
function b(){
}
When JS Engine runs this above code it creates following Execution context (shown in image):
Global Execution Context
Now let's see how JS Engine creates Execution Context (then we will dig out & understand hoisting):
consider this scenario:
b();
console.log(a);
var a = "Hello World!";
function b(){
console.log("Called b!");
}
I can call the function b() even though it is declared later.
This means JS Engine is doing something before my code is executed, lets see what:
JS Engine performs following two steps to while executing any code:
CREATION PHASE :
JS Engine parses - run through your code & identifies variables & functions created by code (which will be used in execution phase)
Setup memory space for Variables & Functions - "Hoisting"
Hoisting - before your code is executed, the JS Engine set asides memory space for Var & Func used inside the code.
These variables & functions comprise the Execution Context of any function that is be executed.
All variables in JS are initially set to undefined.
Execution PHASE: pretty simple to understand,
When the code is executed line-by-line (by JS interpreeter) it can
access the variables defined inside Execution Context
variable assignment are done in this phase
A new Execution Context is created whenever function invocation is there
Execution Context Stack:
What happens when you invoke a function:
function b(){
}
function a(){
b();
}
a();
Now first of all Global Execution Context is going to be created
(as explained above)
then execution starts and interpreeter encounters call to function
a(), and here a new execution context is created pushed on top EC
Stack
so anytime you invoke a function a new EC is created & placed on top of EC Stack.
so now EC for a() is CREATED interpreeter will execute the code
inside a() line-by-line
then intrepreeter encounters call to function b(), this creates
another EC which is pushed on top or EC stack
When b() finishes it will be popped-off the stack then a() will finish &
all the way down to Global EC
see Execution Stack for above code snippet
I have addressed only the topics that are most closely related.
Execution Context is the wrapper around your existing code; which contains code that you have not written; but is generated by the JS Engine.
It comprises of the following -
Global Object
'this'
Outer environment
Your code
An Execution Context is created each time you run your .js file/app. The first step in this creation phase is Hoisting. The JS Engine reserves space or set's up memory for all the variables and functions defined in your code. These are then accessed when your code is executed line-by-line.
For example:
b();
console.log(a);
var a = "hi!";
function b() {
console.log("calling function");
}
Here, the function b() and variable a are both accessed before they are defined, however, due to hoisting the console will not throw any error.
The output will look like - (try it)
calling function
undefined
Notice how the function was executed completely, but we have undefined for the variable. This is because Hoisting is performed differently for functions vs variables. The function as a whole is picked up into the memory, but for the variables, space is reserved as a placeholder with the value of undefined. The actual value is then replaced when the engine executes your code line-by-line.
I hope this clears the concept for you.
I would like to address
Context
this context (relation with context)
Scope
1 : Execution Context
JavaScript is a single threaded language, meaning only one task can be executed at a time. When the JavaScript interpreter initially executes code, it first enters into a global execution context by default. Each invocation of a function from this point on will result in the creation of a new execution context.
This is where confusion often sets in, the term execution context is actually for all intents and purposes referring more to scope and not context. It is an unfortunate naming convention, however it is the terminology as defined by the ECMAScript specification, so we’re kinda stuck with it.
Each time a new execution context is created it is appended to the top of the execution stack. The browser will always execute the current execution context that is atop the execution stack. Once completed, it will be removed from the top of the stack and control will return to the execution context below.
An execution context can be divided into a creation and execution phase. In the creation phase, the interpreter will first create a variable object (also called an activation object) that is composed of all the variables, function declarations, and arguments defined inside the execution context. From there the scope chain is initialized next, and the value of this is determined last. Then in the execution phase, code is interpreted and executed.
2 : this Context
What is “this” Context?
Context is most often determined by how a function is invoked. When a function is called as a method of an object, this is set to the object the method is called on:
var obj = {
foo: function() {
return this;
}
};
obj.foo() === obj; // true
The same principle applies when invoking a function with the new operator to create an instance of an object. When invoked in this manner, the value of this within the scope of the function will be set to the newly created instance:
function foo() {
alert(this);
}
foo() // window
new foo() // foo
When called as an unbound function, this will default to the global context or window object in the browser. However, if the function is executed in strict mode, the context will default to undefined.
3 : Variable scope
A variable can be defined in either local or global scope, which establishes the variables’ accessibility from different scopes during runtime. Any defined global variable, meaning any variable declared outside of a function body will live throughout runtime and can be accessed and altered in any scope. Local variables exist only within the function body of which they are defined and will have a different scope for every call of that function. There it is subject for value assignment, retrieval, and manipulation only within that call and is not accessible outside of that scope.
ECMAScript 6 (ES6/ES2015) introduced the let and const keywords that support the declaration of block scope local variables. This means the variable will be confined to the scope of a block that it is defined in, such as an if statement or for loop and will not be accessible outside of the opening and closing curly braces of the block. This is contrary to var declarations which are accessible outside blocks they are defined in. The difference between let and const is that a const declaration is, as the name implies, constant - a read-only reference to a value. This does not mean the value is immutable, just that the variable identifier cannot be reassigned.
For Other topics:
GC : GC
Prototyping : Prototyping
The "Execution Context" is an umbrella that wraps all of the code to help manage it. It's like a manager that manages any environment. Since there's so many lexical environments because in a JavaScript application you have lots of variables and functions you need a way to manage everything. What comes first, what comes second and so on, and if you did not have an "Execution Context" environment everything goes to hell. So consider the "Execution Context" a wrapper, a manager that manages your code.
When you execute a function you create a new execution context comprising a local memory which is called a variable environment and this which is a placeholder, it will refer inside its execution context to whatever is the left of the . where that function is being called upon.
what is EC(Execution context) in JS ?
Simply put, an execution context is an abstract concept of an
environment where the Javascript code is evaluated and executed.
Whenever any code is run in JavaScript, it’s run inside an execution
context.
For more depth details : https://blog.bitsrc.io/understanding-execution-context-and-execution-stack-in-javascript-1c9ea8642dd0
I suppose a simple example would explain everything.
Note: function.call(object) calls function function in context of object
// HTML
<p id="p"></p>
// JavaScript
function helloWorld() {
alert("HelloWorld: " + this);
}
var p = document.getElementById("p");
helloWorld(); // HelloWorld: [object DOMWindow]
helloWorld.call(window); // HelloWorld: [object DOMWindow]
helloWorld.call("String"); // HelloWorld: String // Note: "toString()"
helloWorld.call(p); // HelloWorld: [object HTMLParagraphElement]
helloWorld.call(new Date()); // HelloWorld: Tue Feb 21 2012 21:45:17 GMT+0100 (Central Europe Standard Time)
Execution Context is a wrapper to help manage the code that is running.
In your code you will see lots of lexical environments (which means areas of the code between { ... }) but which one is currently running is managed via execution contexts. It can contain your code and it also can contain beyond what you've written in your code.
I'm using setTimeout to emulate rendering, and I came to the structure like this:
var Renderer = new Class (
{
Implements: Events,
initialize()
{
this.onRender();
},
onRender: function()
{
// some rendering actions
setTimeout(this.onRender.bind(this), 20);
}
});
Does that code have potential memory leaks because of infinite nesting of closures? Or everything is ok? The only solution I came so far is to rewrite it to usual
function Renderer()
{
var onRender = function()
{
// rendering
setTimeout(onRender, 20);
};
onRender();
};
But I don't want to lose Mootools Events and Classes.
For some reasons I can't use a "singleton" (like window.renderer = new Renderer();) too
Your code is fine, but Andy's answer is misleading because it confuses the scope chain with execution context and, by extension, call stack.
First, setTimeout functions do not execute in the global scope. They still execute in a closure and can access variables from outer scopes. This is because JavaScript uses static scope; that is, the scope chain of a function is defined at the moment that function is created and never changes; the scope chain is a property of the function.
Execution context is different and separate from the scope chain in that it is constructed at the time a function is invoked (whether directly – func(); – or as the result of a browser invocation, such as a timeout expiring). The execution context is composed of the activation object (the function's parameters and local variables), a reference to the scope chain, and the value of this.
The call stack can be thought of as an array of execution contexts. At the bottom of the stack is the global execution context. Each time a function is called, its parameters and this value are stored in a new 'object' on the stack.
If we were to change your onRender function to simply call itself (this.onRender()), the stack would quickly overflow. This is because control would never leave each successive onRender function, allowing its execution context to be popped off the call stack. Instead, we go deeper and deeper with each onRender waiting for the next onRender to return, in an infinite cycle broken only when the stack overflows.
However, with a call to setTimeout, control returns immediately and thus is able to leave the onRender function, causing its execution context to be popped off the stack and discarded (to be freed from memory by the GC).
When the timeout expires, the browser initiates a call to onRender from the global execution context; the call stack is only two deep. There is a new execution context – which by default would inherit the global scope as its this value; that's why you have to bind to your Renderer object – but it still includes the original scope chain that was created when you first defined onRender.
As you can see, you're not creating infinite closures by recursion because closures (scope chains) are created at function definition, not at function invocation. Furthermore, you're not creating infinite execution contexts because they are discarded after onRender returns.
We can make sure you're not leaking memory by testing it. I let it run 500,000 times and didn't observe any leaking memory. Note that the maximum call stack size is around 1,000 (varies by browser), so it's definitely not recursing.
EDIT: This is just a simple example to demontrate the concern I have with a much larger program. I wouldn't use this actual code for anything :)
If I run this -
<!DOCTYPE html>
<html>
<head>
<script>
function update(amount, win, data)
{
win.innerText = 'Count is ' + amount;
setTimeout(function() { update(amount + 1, win, {data: 'something'})}, 1000);
}
window.onload = function() {
var win = document.getElementById('item');
update(0, win, 0);
}
</script>
</head>
<body>
<div id="item"></div>
</body>
</html>
The call to setTimeout presumably creates a closure which captures the contents of the parameters to the "update" function (amount, win, data). So those variables are maintained in memory until the timeout is called and returns so that they will be available inside that function call...
But that function creates a new closure for the next iteration of the timeout... What will be captured in that second closure? Is it just the new copies of those variables or will the ones that formed part of the function call be captured again in the new closure?
Basically will this eventually run out of memory due to the data in each closure getting bigger and bigger, or is this safe and reasonable?
In my understanding, when a closure is created, the current lexical context is bundled with it. In your case, it would be the amount, win, data.
This context will be used, when the timeout fires, to execute the closure and thus call once again the function update; this call, although it might appear so, is not recursive, because the previous execution of update already ended and its original context (dynamic, which is different from the lexical one) has already been freed. (I think this is important to notice, because it seems you are worrying about a sort of stack growth due to recursion).
So, again, update is executed a second time and again a timeout is set and a closure created. This closure is bundled with the current lexical context of execution (which still includes just amount, win, data) and scheduled with the timer. then update finishes and removed from the stack. then again the timer fires and update is called again...
So, you should not worry about an unlimited growth of the context, for two reasons: first, only the lexical context is bundled with the closure; the call is not actually recursive.
A new closure is created every time the timeout callback is called, as you correctly say. But once the callback has been executed, there is no longer anything referencing the previous closure, so it can be garbage collected.