Closure in javascript. Closures 2018-07-09

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Closures

closure in javascript

One solution in this case is to use more closures: in particular, to use a function factory as described earlier: function showHelp help { document. Thanks to JavaScript's lexical scoping, they each have access to the privateCounter variable and changeBy function. One other way to write the above using anonymous closures is: function showHelp help { document. The displayName function has no local variables of its own. Because the loop has already run its course by that time, the item variable object shared by all three closures has been left pointing to the last entry in the helpText list. Consequently, you can use a closure anywhere that you might normally use an object with only a single method. The instance of displayName maintains a reference to its lexical environment, within which the variable name exists.

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Closures

closure in javascript

In this case, myFunc is a reference to the instance of the function displayName created when makeFunc is run. This has obvious parallels to object-oriented programming, where objects allow us to associate some data the object's properties with one or more methods. If you try this code out, you'll see that it doesn't work as expected. Neither of these private items can be accessed directly from outside the anonymous function. Here, though, we create a single lexical environment that is shared by three functions: counter. The loop cycles through these definitions, hooking up an onfocus event to each one that shows the associated help method.


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Closures

closure in javascript

This environment consists of any local variables that were in-scope at the time the closure was created. We could store this function in a separate variable makeCounter and use it to create several counters. Rather than the callbacks all sharing a single lexical environment, the makeHelpCallback function creates a new lexical environment for each callback, in which help refers to the corresponding string from the helpText array. Each time one of the counters is called, its lexical environment changes by changing the value of this variable; however changes to the variable value in one closure do not affect the value in the other closure. The reason is that functions in JavaScript form closures. Private methods aren't just useful for restricting access to code: they also provide a powerful way of managing your global namespace, keeping non-essential methods from cluttering up the public interface to your code. However, the same local variables in displayName will be used if they exist.

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Closures

closure in javascript

In this context, we can say all closures have access to all outer function scopes within which they were declared. JavaScript does not provide a native way of doing this, but it is possible to emulate private methods using closures. The lexical environment contains two private items: a variable called privateCounter and a function called changeBy. However, because the code still works as expected, this is obviously not the case in JavaScript. The reason is that whenever the constructor is called, the methods would get reassigned that is, for every object creation. The following code illustrates how to use closures to define public functions that can access private functions and variables.

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Closures

closure in javascript

Each closure references a different version of the privateCounter variable through its own closure. A closure is the combination of a function and the lexical environment within which that function was declared. Situations where you might want to do this are particularly common on the web. This is an example of lexical scoping, which describes how a parser resolves variable names when functions are nested. Here's the JavaScript: function makeSizer size { return function { document. The function it returns takes a single argument, y, and returns the sum of x and y. They share the same function body definition, but store different lexical environments.

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Closures

closure in javascript

In essence, makeAdder is a function factory — it creates functions which can add a specific value to their argument. No matter what field you focus on, the message about your age will be displayed. The shared lexical environment is created in the body of an anonymous function, which is executed as soon as it has been defined. One way of doing this is to specify the font-size of the body element in pixels, then set the size of the other elements on the page such as headers using the relative em unit: body { font-family: Helvetica, Arial, sans-serif; font-size: 12px; } h1 { font-size: 1. At first glance, it may seem unintuitive that this code still works.

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Closures

closure in javascript

Practical closures Closures are useful because they let you associate some data the lexical environment with a function that operates on that data. Our code is generally attached as a callback: a single function which is executed in response to the event. Using closures in this way provides a number of benefits that are normally associated with object-oriented programming -- in particular, data hiding and encapsulation. Much of the code we write in front-end JavaScript is event-based — we define some behavior, then attach it to an event that is triggered by the user such as a click or a keypress. The reason for this is that the functions assigned to onfocus are closures; they consist of the function definition and the captured environment from the setupHelp function's scope. The following example instead appends to the existing prototype: function MyObject name, message { this.

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Closures

closure in javascript

In add5's lexical environment, x is 5, while in the lexical environment for add10, x is 10. You'll notice we're defining an anonymous function that creates a counter, and then we call it immediately and assign the result to the counter variable. Three closures have been created by the loop, but each one shares the same single lexical environment, which has a variable with changing values item. Another alternative could be to use forEach to iterate over the helpText array and attach a listener to each , as shown: function showHelp help { document. Nested functions have access to variables declared in their outer scope. Consider the following example: Helpful notes will appear here E-mail: Name: Age: function showHelp help { document. In some programming languages, the local variables within a function exist only for the duration of that function's execution.

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Closures

closure in javascript

Those three public functions are closures that share the same environment. The displayName function is an inner function that is defined inside init and is only available within the body of the init function. Instead, they must be accessed by the three public functions that are returned from the anonymous wrapper. . We can attach them to buttons in this case links as follows: document. However, because inner functions have access to the variables of outer functions, displayName can access the variable name declared in the parent function, init. In the above example we use our function factory to create two new functions — one that adds 5 to its argument, and one that adds 10.

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Closures

closure in javascript

Consider the following case: function MyObject name, message { this. Once makeFunc has finished executing, you might expect that the name variable would no longer be accessible. For instance, suppose we wish to add some buttons to a page that adjust the text size. . .


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