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Javascript Closures

Javascript Closures
Introduction Closure A "closure" is an expression (typically a function) that can have free variables together with an environment that binds those variables (that "closes" the expression). Closures are one of the most powerful features of ECMAScript (javascript) but they cannot be property exploited without understanding them. The simple explanation of a Closure is that ECMAScript allows inner functions; function definitions and function expressions that are inside the function bodes of other functions. Unfortunately, properly understanding closures requires an understanding of the mechanism behind them, and quite a bit of technical detail. The Resolution of Property Names on Objects ECMAScript recognises two categories of object, "Native Object" and "Host Object" with a sub-category of native objects called "Built-in Object" (ECMA 262 3rd Ed Section 4.3). Assignment of Values var objectRef = new Object(); A property with the name "testNumber" can be created as:- Reading of Values Finally:- Related:  JavaScript

Truth, Equality and JavaScript | JavaScript, JavaScript... You don’t have to be a JavaScript novice to get confused by this… or this… The good news is that there is a standard and all browsers follow it. Some authors will tell you to fear coercion and and code against it. I hope to persuade you that coercion is a feature to be leveraged (or at the very least understood), not avoided… Is x true? Does x equal y? Conditionals In JavaScript, all conditional statements and operators follow the same coercion paradigm. The construct if ( Expression ) Statement will coerce the result of evaluating the Expression to a boolean using the abstract method ToBoolean for which the ES5 spec defines the following algorithm: Now we can see why, in the introductory example, if ([0]) allows entry to the subsequent block: an array is an object and all objects coerce to true. Here’s a few more examples. The Equals Operator (==) The == version of equality is quite liberal. Anyway, rant over, lets take a look at the way ECMA defines how == works. [0] == true; Like this:

Summary of JavaScript closures Summary of JavaScript closures If everything seems completely unclear then the best thing to do is to play with the examples. Reading an explanation is much harder than understanding examples. Final points: Whenever you use function inside another function, a closure is used. Links TrimBreakpoint is a tricky use of closures to let you inspect local variables for a function from a popup breakpoint window. JavaScript Scoping and Hoisting Do you know what value will be alerted if the following is executed as a JavaScript program? var foo = 1; function bar() { if (!foo) { var foo = 10; } alert(foo); } bar(); If it surprises you that the answer is “10”, then this one will probably really throw you for a loop: var a = 1; function b() { a = 10; return; function a() {} } b(); alert(a); Here, of course, the browser will alert “1”. Scoping in JavaScript One of the sources of most confusion for JavaScript beginners is scoping. #include <stdio.h> int main() { int x = 1; printf("%d, ", x); // 1 if (1) { int x = 2; printf("%d, ", x); // 2 } printf("%d\n", x); // 1 } The output from this program will be 1, 2, 1. var x = 1; console.log(x); // 1 if (true) { var x = 2; console.log(x); // 2 } console.log(x); // 2 In this case, Firebug will show 1, 2, 2. To a lot of programmers who are used to languages like C, C++, C#, or Java, this is unexpected and unwelcome. Declarations, Names, and Hoisting function foo() { bar(); var x = 1; }

JavaScript Closures 101- they&#039;re not magic JavaScript Closures 101- they're not magic Credits: This tutorial is written by Morris Johns. Morris maintains a personal blog called Developing Thoughts where he can be contacted via. This tutorial explains closures so that a regular programmer can understand them - using working JavaScript code. Closures are not hard to understand once the core concept is grokked. This article is intended for programmers with some programming experience in a main-stream language, and who can read the following JavaScript function: An Example of a Closure Two one sentence summaries: a closure is the local variables for a function - kept alive after the function has returned, or a closure is a stack-frame which is not deallocated when the function returns. The following code returns a reference to a function: Most JavaScript programmers will understand how a reference to a function is returned to a variable in the above code.

JavaScript Garden Although JavaScript deals fine with the syntax of two matching curly braces for blocks, it does not support block scope; hence, all that is left in the language is function scope. function test() { // a scope for(var i = 0; i < 10; i++) { // not a scope // count } console.log(i); // 10} There are also no distinct namespaces in JavaScript, which means that everything gets defined in one globally shared namespace. Each time a variable is referenced, JavaScript will traverse upwards through all the scopes until it finds it. In the case that it reaches the global scope and still has not found the requested name, it will raise a ReferenceError. The Bane of Global Variables // script Afoo = '42'; // script Bvar foo = '42' The above two scripts do not have the same effect. Again, that is not at all the same effect: not using var can have major implications. // global scopevar foo = 42;function test() { // local scope foo = 21;}test();foo; // 21 Local Variables var foo = 3; bar = 4;}test(10); Hoisting

Closures (Lambda Expressions) for the Java Programming Language Want to learn JavaScript in 2015 / 2016? Now you could choose to continue reinforcing what you have already learnt or find new things to learn. I personally took a shine to Functional programming along with a library called RxJS by author and Sourcerer Matt Podwysocki. This all happened from taking a course presented by Jafar Husain, he is a very genuinely enthusiastic teacher. That to me is when I learn the best, if you are watching/interacting with a person who loves what he is doing it shows and you naturally become more interested. Unfortunately the course I’m talking about is due for release on FrontendMasters as I took it live a couple of months ago and it takes that time to get up onto the site. For more knowledge on RxJS Ben Lesh’s tutorials on are an insightful way to get your teeth into RxJS, there a couple of new ones from André Staltz. I should mention that RxJs have a great gitter channel too, you’ll need Github to join but they are a really helpful bunch.

10 Interview Questions Every JavaScript Developer Should Know — JavaScript Scene Good to hear: Classes: create tight coupling or hierarchies/taxonomies.Prototypes: mentions of concatenative inheritance, prototype delegation, functional inheritance, object composition. Red Flags: No preference for prototypal inheritance & composition over class inheritance. Learn More: 4. OOP Pros: It’s easy to understand the basic concept of objects and easy to interpret the meaning of method calls. OOP Cons: OOP Typically depends on shared state. FP Pros: Using the functional paradigm, programmers avoid any shared state or side-effects, which eliminates bugs caused by multiple functions competing for the same resources. FP also tends to favor declarative and denotational styles, which do not spell out step-by-step instructions for operations, but instead concentrate on what to do, letting the underlying functions take care of the how. Red flags: 5. This is a trick question. Rarely, almost never, or never. Any other response. 6. There is more than one type of prototypal inheritance: 7. 8.

all this Coming from a sane language you might think that this in JavaScript is kind of like this in an object oriented language like Java, something that refers to values stored in instance properties. Not so. In JavaScript it is best to think of the this as a boggart carrying a bag of data with an undetectable extension charm. What follows is what I would want my co-workers to know about using this in JavaScript. global this In a browser, at the global scope, this is the window object. <script type="text/javascript"> console.log(this === window); //true</script> jsfiddle In a browser, using var at the global scope is the same as assigning to this or window. <script type="text/javascript"> var foo = "bar"; console.log(; //logs "bar" console.log(; //logs "bar"</script> jsfiddle If you create a new variable without using var or let (ECMAScript 6) you are adding or changing a property on the global this. jsfiddle In node using the repl, this is the top namespace. test.js: function this