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Read "JavaScript The Complete Reference 3rd Edition" by Thomas Powell available from Rakuten Kobo. Sign up today and get $5 off your first purchase. JavaScript The Complete Reference,. Second Edition. Second Edition. Thomas Powell. Fritz Schneider. McGraw-Hill/Osborne. New York Chicago San. JavaScript The Complete Reference 3rd Edition (3rd ed.) (The Complete Reference series) by Thomas A. Powell. Read online, or download in secure PDF or.

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JavaScript The Complete Reference, Second Edition Similar Free eBooks The Complete Reference, C++: McGraw-Hill has no responsibility for the. JavaScript The Complete Reference 3rd Edition. eBook Details: Paperback: pages; Publisher: WOW! eBook; 3rd edition (August 14, ). The JavaScript Handbook follows the 80/20 rule: learn 80% of We have a complete guide dedicated to ECMAScript where you can dive into it.

Resilient Web Design By Jeremy Keith HTML Rather than being a guide to solid web design, Resilient Web Design has been written to highlight some of the approaches and techniques to web design that have proven to be resilient over the years. It includes chapters on the box model, layout, tables, color, typography, and transitions, and does include interactive examples.

Please note that you will need to signup to download this ebook. The book has been organized into 2 sections: First, there are the reading portions where it describes how an API works with interactive examples , and then there are the hands-on lessons that will walk you through the process of building your own canvas apps. The free online version includes 53 interactive examples and exercises. It has been written for designers and developers that are looking to quickly add SVG to their workflow.

Typography Handbook Typography Handbook is a concise, referential guide on best web typographic practices. If you want to keep your code efficient, more manageable, and up-to-date with the latest best practices, this book is for you. Each chapter covers a concept and gives you the chance to try example code related to the concept. Programming JavaScript Applications By Eric Elliott In this early release edition of Programming JavaScript Applications , you will have the chance to take your existing JavaScript skills to the next level and learn how to build complete web scale or enterprise applications that are easy to extend and maintain.

Developing Backbone. This must stop. It is available as a PDF file, so you can easily download it and use it as per your convenience. In any case, the prototype of car is Object: If you initialize an array, which is an object: The prototype is Array. You can verify this by checking the Object. All the properties and methods of the prototype are available to the object that has that prototype: The above example you saw is an example of the prototype chain at work.

I can make an object that extends Array and any object I instantiate using it, will have Array and Object in its prototype chain and inherit properties and methods from all the ancestors. In addition to using the new operator to create an object, or using the literals syntax for objects and arrays, you can instantiate an object using Object.

The first argument passed is the object used as prototype: You can check the prototype of an object using the isPrototypeOf method: Pay attention because you can instantiate an array using.

Before that, JavaScript only had a quite unique way to implement inheritance. Its prototypal inheritance, while in my opinion great, was different from any other popular programming language. People coming from Java or Python or other languages had a hard time understanding the intricacies of prototypal inheritance, so the ECMAScript committee decided to introduce a syntactic sugar on top of them, and resemble how classes-based inheritance works in other popular implementations.

This is important: JavaScript under the hoods is still the same, and you can access an object prototype in the usual way. This is how a class looks. A class has an identifier, which we can use to create new objects using new ClassIdentifier.

When the object is initialized, the constructor method is called, with any parameters passed. A class also has as many methods as it needs. In this case hello is a method and can be called on all objects derived from this class: A class can extend another class, and objects initialized using that class inherit all the methods of both classes. If the inherited class has a method with the same name as one of the classes higher in the hierarchy, the closest method takes precedence: I am a programmer.

Inside a class, you can reference the parent class calling super. Normally methods are defined on the instance, not on the class. Static methods are executed on the class instead: JavaScript does not have a built-in way to define private or protected methods.

You can add methods prefixed with get or set to create a getter and setter, which are two different pieces of code that are execute based on what you are doing: If you only have a getter, the property cannot be set, and any attempt at doing so will be ignored: If you only have a setter, you can change the value but not access it from the outside: When the code runs into an unexpected problem, the idiomatic JavaScript way to handle this situation is through exceptions.

An exception is created using the throw keyword: Where value can be any JavaScript value including a string, a number or an object. As soon as JavaScript executes this line, the normal program flow is halted and the control is held back to the nearest exception handler.

Any exception raised in the lines of code included in the try block is handled in the corresponding catch block: You can add multiple handlers, that can catch different kinds of errors. To complete this statement JavaScript has another statement called finally , which contains code that is executed regardless of the program flow, if the exception was handled or not, if there was an exception or if there wasn't: You can use finally without a catch block, to serve as a way to clean up any resource you might have opened in the try block, like files or network requests: If an exception is raised in the inner try , it's handled in the outer catch block.

JavaScript semicolons are optional. I personally like to avoid using semicolons in my code, but many people prefer them. Semicolons in JavaScript divide the community.

Some prefer to use them always, no matter what. Others like to avoid them. After using semicolons for years, in the fall of I decided to try avoiding them as needed, and I did set up Prettier to automatically remove semicolons from my code, unless there is a particular code construct that requires them.

This is all possible because JavaScript does not strictly require semicolons. When there is a place where a semicolon was needed, it adds it behind the scenes. The process that does this is called Automatic Semicolon Insertion. The JavaScript parser will automatically add a semicolon when, during the parsing of the source code, it finds these particular situations: Based on those rules, here are some examples. Take this: Cannot read property 'forEach' of undefined because based on rule 1 JavaScript tries to interpret the code as.

Such a piece of code: Instead, the above code raises a TypeError: Another example based on rule 4: Instead, it's undefined , because JavaScript inserts a semicolon after return. Instead you should put the opening bracket right after return: Be careful. Some people are very opinionated on semicolons. We just need to pay a bit of attention, even if most of the times those basic scenarios never show up in your code.

Pick some rules: And ultimately, always test your code to make sure it does what you want. JavaScript allows you to use 3 types of quotes: The first 2 are essentially the same: The only difference lies in having to escape the quote character you use to delimit the string: There are various style guides that recommend always using one style vs the other.

I personally prefer single quotes all the time, and use double quotes only in HTML. Backticks are a recent addition to JavaScript, since they were introduced with ES6 in They have a unique feature - they allow multiline strings. Multiline strings are also possible using regular strings, using escape characters: Using backticks the key at the left of the number 1 on the keyboard , you can avoid using an escape character: Not just that. Those are called Template Literals.

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Introduced in ES, aka ES6, Template Literals offer a new way to declare strings, but also some new interesting constructs which are already widely popular. The syntax at a first glance is very simple, just use backticks instead of single or double quotes: They are unique because they provide a lot of features that normal strings built with quotes, in particular: Template literals make multiline strings much simpler.

Keep in mind that space is meaningful, so doing this: Is going to create a string like this: An easy way to fix this problem is by having an empty first line, and appending the trim method right after the closing backtick, which will eliminate any space before the first character: Template literals provide an easy way to interpolate variables and expressions into strings.

The styled. This function returns a string, which can be the result of any kind of computation. The first is something , the string until the first interpolation, and the second is an empty string, the space between the end of the first interpolation we only have one and the end of the string.

The function that is passed those values can do anything with them, and this is the power of this kind of feature. The most simple example is replicating what the string interpolation does, by simply joining literals and expressions: A function is a block of code, self contained, that can be defined once and run any number of times you want.

Functions in JavaScript are objects , a special kind of object: Their superpower lies in the fact that they can be invoked. In addition, functions are said to be first class functions because they can be assigned to a value, and they can be passed as arguments and used as a return value.

Functions can be assigned to variables this is called a function expression: Named function expressions are similar, but play nicer with the stack call trace, which is useful when an error occurs - it holds the name of the function: ES introduced trailing commas for parameters, a feature that helps reducing bugs due to missing commas when moving around parameters e. You can wrap all your arguments in an array, and use the spread operator when calling the function: With many parameters, remembering the order can be difficult.

Using objects, destructuring allows to keep the parameter names: Any function is terminated when its lines of code end, or when the execution flow finds a return keyword. When JavaScript encounters this keyword it exits the function execution and gives control back to its caller. To simulate returning multiple values, you can return an object literal , or an array , and use a destructuring assignment when calling the function.

The nested function is scoped to the outside function, and cannot be called from the outside. Consider this example: This is because the handling of this is different in the two functions declarations style. This implies that arrow functions are not suitable to be used for object methods and constructors arrow function constructors will actually raise a TypeError when called.

Functions in particular are moved at the top of their scope. Internally, JavaScript moves the function before its call, along with all the other functions found in the same scope: The variable declaration is hoisted, but not the value, so not the function. The same happens for let declarations. This is because var declarations are hoisted and initialized with undefined as a value, while const and let are hoisted but not initialized.

They slightly differ from regular functions. I already introduced arrow functions above, but they are so important they need their own section. In my opinion this change was so welcome that you now rarely see in modern codebases the usage of the function keyword. If the function body contains just a single statement, you can omit the parentheses and write all on a single line: If you have one and just one parameter, you could omit the parentheses completely: Thanks to this short syntax, arrow functions encourage the use of small functions.

Arrow functions allow you to have an implicit return: Another example, returning an object remember to wrap the curly brackets in parentheses to avoid it being considered the wrapping function body brackets: When defined as a method of an object, in a regular function this refers to the object, so you can do: The this scope with arrow functions is inherited from the execution context.

An arrow function does not bind this at all, so its value will be looked up in the call stack, so in this code car.

Arrow functions cannot be used as constructors as well, when instantiating an object. It will raise a TypeError. This is where regular functions should be used instead, when dynamic context is not needed.

This is also a problem when handling events. DOM Event listeners set this to be the target element, and if you rely on this in an event handler, a regular function is necessary: As you can see, the state of the variable say is linked to the function that's returned from prepareBark. Also notice that we redefine a new say variable the second time we call prepareBark , but that does not affect the state of the first prepareBark scope. This is how a closure works: This section aims to explain what you should use, as of Iterates a and builds a new array with the result of executing f on each a element.

Iterates a and builds a new array with elements of a that returned true when executing f on each a element. If initialValue is specified, accumulator in the first iteration will equal to that value.

Iterates a , can be stopped using return or break and an iteration can be skipped using continue. You can also detect if the iteration ended by looking at it. Returns true if a contains value after the position i. Returns a string concatenation of the array elements. Pass a parameter to add a custom separator: JavaScript provides many ways to iterate through loops. This section explains all the various loop possibilities in modern JavaScript with a small example and the main properties.

Introduced in ES5. Given an array, you can iterate over its properties using list.

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The difference with do ES introduced the for Notice the use of const. This loop creates a new scope in every iteration, so we can safely use that instead of let. JavaScript in the browser uses an event-driven programming model. Everything starts by following an event. This section provides an introduction to JavaScript events and how event handling works. The event could be the DOM is loaded, or an asynchronous request that finishes fetching, or a user clicking an element or scrolling the page, or the user types on the keyboard.

You can register multiple handlers for the same event, and they will all be called when that event happens. This style of event handlers is very rarely used today, due to its constrains, but it was the only way in the early days of JavaScript: This is common when an object has at most one event handler, as there is no way to add multiple handlers in this case: This is the modern way.

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You can listen on window to intercept "global" events, like the usage of the keyboard, and you can listen on specific elements to check events happening on them, like a mouse click on a button. Other properties are provided by specific kind of events, as Event is an interface for different specific events: For example when a KeyboardEvent happens, you can check which key was pressed, in a readable format Escape , Enter and so on by checking the key property: You want to track when users click on the button, and you have two event listeners, one on button , and one on container.

Bubbling means that the event propagates from the item that was clicked the child up to all its parent tree, starting from the nearest one.

In our example, the handler on button will fire before the container handler. Capturing is the opposite: You can choose to adopt event capturing by applying a third argument to addEventListener, setting it to true: The order follows this principle: While doing so, it calls any event handler associated with the event capturing phase.

Once it reaches the target, it then repeats the journey up to the parents tree until the Window object, again calls the event handlers bubbling phase. A click event on a will propagate to section and then body. You can stop the propagation by calling the stopPropagation method of an Event, usually at the end of the event handler: Of course in this case click is fired just before this event. Be careful with mousemove , as it fires many times during the mouse movement see throttling later. The scroll event is fired on window every time you scroll the page.

Inside the event handler you can check the current scrolling position by checking window. Keep in mind that this event is not a one-time thing. As we mentioned above, mousemove and scroll are two events that are not fired one-time per event, but rather they continuously call their event handler function during all the duration of the action.

If you perform a complex operation in the event handler, you will affect the performance and cause a sluggish experience to your site users.

A simple and easy to understand implementation is this, which uses setTimeout to cache the scroll event every ms: This section aims to explain the inner details of how JavaScript works with a single thread, and how it handles asynchronous functions.

Your JavaScript code runs single threaded. There is just one thing happening at a time. You just need to pay attention to how you write your code and avoid anything that could block the thread, like synchronous network calls or infinite loops.

In general, in most browsers there is an event loop for every browser tab, to make every process isolated and avoid a web page with infinite loops or heavy processing to block your entire browser. The environment manages multiple concurrent event loops, to handle API calls for example. Web Workers run in their own event loop as well.

You mainly need to be concerned that your code will run on a single event loop, and write code with this thing in mind to avoid blocking it. Any JavaScript code that takes too long to return back control to the event loop will block the execution of any JavaScript code in the page, even block the UI thread, and the user cannot click around, scroll the page, and so on. Network requests, Node. While doing so, it adds any function call it finds, to the call stack and executes each one in order.

You know the error stack trace you might be familiar with, in the debugger or in the browser console? The browser looks up the function names in the call stack to inform you which function originates the current call: When this code runs, first foo is called. Inside foo we first call bar , then we call baz.

JavaScript finds things to execute, runs them in order. Inside foo we first call setTimeout, passing bar as an argument, and we instruct it to run immediately as fast as it can, passing 0 as the timer. Then we call baz. When setTimeout is called, the Browser or Node. Once the timer expires, in this case immediately as we put 0 as the timeout, the callback function is put in the Message Queue.

The Message Queue is also where user-initiated events like click or keyboard events, or fetch responses are queued before your code has the opportunity to react to them. Or also DOM events like onLoad. For example, if you set the setTimeout timeout to 2 seconds, you don't have to wait 2 seconds - the wait happens elsewhere.

Promises that resolve before the current function ends will be executed right after the current function. I find the analogy of a rollercoaster ride at an amusement park nicely explains it: JavaScript is synchronous by default, and is single threaded.

This means that code cannot create new threads and run in parallel. In the current consumer computers, every program runs for a specific time slot, and then it stops its execution to let another program continue its execution. When a program is waiting for a response from the network, it cannot halt the processor until the request finishes.

Normally, programming languages are synchronous, and some provide a way to manage asynchronicity, in the language or through libraries. Some of them handle async by using threads, spawning a new process.

JavaScript is synchronous by default and is single threaded. But JavaScript was born inside the browser, its main job, in the beginning, was to respond to user actions, like onClick , onMouseOver , onChange , onSubmit and so on.

How could it do this with a synchronous programming model? The answer was in its environment. The browser provides a way to do it by providing a set of APIs that can handle this kind of functionality.

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This event handler accepts a function, which will be called when the event is triggered: We can do this because JavaScript has first-class functions, which can be assigned to variables and passed around to other functions called higher-order functions. XHR requests also accept a callback. In this example, by assigning a function to a property that will be called when a particular event occurs in this case, the state of the request changes: How do you handle errors with callbacks?

One very common strategy is to use what Node. If there is no error, the object is null. If there is an error, it contains some description of the error and other information.

However every callback adds a level of nesting, and when you have lots of callbacks, the code starts to be complicated very quickly: Starting with ES6, JavaScript introduced several features that help us with asynchronous code that do not involve using callbacks: Promises are one way to deal with asynchronous code in JavaScript, without writing too many callbacks in your code. A promise is commonly defined as a proxy for a value that will eventually become available.

Promises are one way to deal with asynchronous code, without writing too many callbacks in your code. Although being around since years, they have been standardized and introduced in ES, and now they have been superseded in ES by async functions. Once a promise has been called, it will start in pending state. This means that the caller function continues the execution, while it waits for the promise to do its own processing, and give the caller function some feedback.

At this point, the caller function waits for it to either return the promise in a resolved state , or in a rejected state , but the function continues its execution while the promise does its work. In addition to your own code and libraries code, promises are used by standard modern Web APIs such as: As you can see the promise checks the done global constant, and if that's true, we return a resolved promise, otherwise a rejected promise.

Using resolve and reject we can communicate back a value, and in the above case we just return a string, but it could be an object as well. Running checkIfItsDone will execute the isItDoneYet promise and will wait for it to resolve, using the then callback, and if there is an error, it will handle it in the catch callback. The Fetch API is a promise-based mechanism, and calling fetch is equivalent to defining our own promise using new Promise. In this example, we call fetch to get a list of TODO items from the todos.

Running fetch returns a response , which has many properties, and within those we reference: So given those premises, this is what happens: This operation will cause the promise chain to skip all the chained promises listed and will skip directly to the catch statement at the bottom, logging the Request failed text along with the error message.

If that succeeds instead, it calls the json function we defined. Since the previous promise, when successful, returned the response object, we get it as an input to the second promise. In the example, in the previous section, we had a catch that was appended to the chain of promises.

When anything in the chain of promises fails and raises an error or rejects the promise, the control goes to the nearest catch statement down the chain. If inside the catch you raise an error, you can append a second catch to handle it, and so on. If you need to synchronize different promises, Promise. You are not limited to using fetch of course, any promise is good to go. Async functions are a combination of promises and generators, and basically, they are a higher level abstraction over promises.

Let me repeat: When Promises were introduced in ES, they were meant to solve a problem with asynchronous code, and they did, but over the 2 years that separated ES and ES, it was clear that promises could not be the final solution. Promises were introduced to solve the famous callback hell problem, but they introduced complexity on their own, and syntax complexity.

They were good primitives around which a better syntax could be exposed to the developers, so when the time was right we got async functions. When you want to call this function you prepend await , and the calling code will stop until the promise is resolved or rejected. One caveat: Here's an example: Prepending the async keyword to any function means that the function will return a promise. As you can see in the example above, our code looks very simple.

Compare it to code using plain promises, with chaining and callback functions. And this is a very simple example, the major benefits will arise when the code is much more complex. Async functions can be chained very easily, and the syntax is much more readable than with plain promises: Debugging promises is hard because the debugger will not step over asynchronous code.

There is one feature of JavaScript that might cause a few headaches to developers, related to loops and scoping. It basically iterates and for 5 times it adds a function to an array called operations.

This function simply console logs the loop index variable i. Since var declarations are hoisted , the above code equals to. So, in the for-of loop, i is still visible, it's equal to 5 and every reference to i in the function is going to use this value.

The simplest solution is to use let declarations. Introduced in ES, they are a great help in avoiding some of the weird things about var declarations. Simply changing var to let in the loop variable is going to work fine: How is this possible? This works because on every loop iteration i is created as a new variable each time, and every function added to the operations array gets its own copy of i.

Keep in mind you cannot use const in this case, because there would be an error as for tries to assign a new value in the second iteration. In this case you can wrap the entire function and bind i to it. Since in this way you're creating a function that immediately executes, you return a new function from it, so we can execute it later: When writing JavaScript code, you might want to delay the execution of a function.

This is the job of setTimeout. You specify a callback function to execute later, and a value expressing how later you want it to run, in milliseconds: This syntax defines a new function. You can call whatever other function you want in there, or you can pass an existing function name, and a set of parameters: This is generally not used, but you can store this id, and clear it if you want to delete this scheduled function execution: If you specify the timeout delay to 0 , the callback function will be executed as soon as possible, but after the current function execution: This is especially useful to avoid blocking the CPU on intensive tasks and let other functions execute while performing a heavy calculation, by queuing functions in the scheduler.

The function above runs every 2 seconds unless you tell it to stop, using clearInterval , passing it the interval id that setInterval returned: For example this code runs something unless App.

Maybe the function takes different execution times, depending on network conditions for example: To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes: Outside any object, this in strict mode is always undefined.

Notice I mentioned strict mode. In this case, using a regular function, this is automatically bound to the object. Using call or apply , at the function invocation step: The first parameter you pass to call or apply is always bound to this. The difference between call and apply is just that the second one wants an array as the arguments list, while the first accepts a variable number of parameters, which passes as function arguments. In event handlers callbacks, this refers to the HTML element that received the event: And in a different way , as enabling Strict Mode changes the semantics of the JavaScript language.

Strict Mode mostly removes functionality that was possible in ES3, and deprecated since ES5 but not removed because of backwards compatibility requirements.

Strict mode is optional. So we have the 'use strict' directive we need to use to enable Strict Mode. You can put it at the beginning of a file, to apply it to all the code contained in the file: You can also enable Strict Mode for an individual function, by putting 'use strict' at the beginning of the function body: If you assign a value to an undeclared variable, JavaScript by default creates that variable on the global object: The same applies to Infinity, NaN, eval , arguments and more.

Also, sloppy mode allows to set properties on primitive values, without failing, but also without doing nothing at all: In sloppy mode, if you try to delete a property that you cannot delete, JavaScript simply returns false, while in Strict Mode, it raises a TypeError: Octal syntax in Strict Mode is disabled. By default, prepending a 0 to a number compatible with the octal numeric format makes it sometimes confusingly interpreted as an octal number: You can still enable octal numbers in Strict Mode using the 0oXX syntax: Strict Mode disables the with keyword, to remove some edge cases and allow more optimization at the compiler level.

An Immediately-invoked Function Expression is a way to execute functions immediately, as soon as they are created. We basically have a function defined inside parentheses, and then we append to execute that function: Those wrapping parentheses are actually what make our function, internally, be considered an expression.

You could also put the invoking parentheses inside the expression parentheses, there is no difference, just a styling preference: An IIFE can also be named regular functions not arrow functions. This prevents issues when blindly concatenating two JavaScript files. Since JavaScript does not require semicolons, you might concatenate with a file with some statements in its last line that causes a syntax error.

Performing math operations and calculus is a very common thing to do with any programming language. JavaScript offers several operators to help us work with numbers. If you divide by zero, JavaScript does not raise any error but returns the Infinity value or -Infinity if the value is negative.

A reminder by zero is always NaN , a special value that means "Not a Number": Increment a number. This is a unary operator, and if put before the number, it returns the value incremented. If the operand is not a number, it tries to convert it. Otherwise if the operand is already a number, it does nothing.

The result is 2. What operations are executed first, and which need to wait? Some operations have more precedence than the others. The precedence rules are listed as follows: The Math object contains lots of utilities math-related. While Node. Every major decision such as a module system must be first standardized by ECMAScript and then implemented by the browser.

This standardization process completed with ES6 and browsers started implementing this standard trying to keep everything well aligned, working all in the same way, and now ES Modules are supported in Chrome, Safari, Edge and Firefox since version Modules are very cool, because they let you encapsulate all sorts of functionality, and expose this functionality to other JavaScript files as libraries.

A module is a JavaScript file that exports one or more value objects, functions or variables , using the export keyword. For example, this module exports a function that returns a string uppercase: In this example, the module defines a single, default export , so it can be an anonymous function. Otherwise it would need a name to distinguish it from other exports. Now, any other JavaScript module can import the functionality offered by uppercase.

In this example, the uppercase. You can also use an absolute path for the module import, to reference modules defined on another domain: This creates one default export. In a file however you can export more than one thing, by using this syntax: You can import the default export, and any non-default export by name, like in this common React import: You can check an ES Modules example on https: Modules are fetched using CORS.

This means that if you reference scripts from other domains, they must have a valid CORS header that allows cross-site loading like Access-Control-Allow-Origin: ES Modules are one of the biggest features introduced in modern browsers. They are part of ES6 but the road to implement them has been long. We can now use them! Webpack is probably going to still be a huge player even if ES Modules land in the browser, but having such a feature directly built in the language is huge for a unification of how modules work in the client-side and on Node.

The CommonJS module specification is the standard used in Node. They let you create clearly separate and reusable snippets of functionality, each testable on its own. In CommonJS, modules are loaded synchronously, and processed in the order the JavaScript runtime finds them. This system was born with server-side JavaScript in mind, and is not suitable for the client-side this is why ES Modules were introduced.

A JavaScript file is a module when it exports one or more of the symbols it defines, being them variables, functions, objects: A simple example can be found in this Glitch. To end with, a guide to a few terms used in frontend development that might be alien to you. Code is asynchronous when you initiate something, forget about it, and when the result is ready you get it back without having to wait for it.


The typical example is an AJAX call, which might take even seconds and in the meantime you complete other stuff, and when the response is ready, the callback function gets called. An if statement contains a block, a for loop contains a block. With Function Scoping, any variable defined in a block is visible and accessible from inside the whole block, but not outside of it. A declarative approach is when you tell the machine what you need to do, and you let it figure out the details.

React is considered declarative, as you reason about abstractions rather than editing the DOM directly. Every high level programming language is more declarative than a low level programming language like Assembler.

JavaScript is more declarative than C. HTML is declarative. For example a user that browses with JavaScript disabled should be able to have a fallback to a plain HTML version of the page.

Or for a browser that has not implemented an API, you should have a fallback to avoid completely breaking the experience of the user. With Function Scoping, any variable defined in a function is visible and accessible from inside the whole function.

A mutable variable can be changed. The same applies to objects and arrays. Lexical Scoping is a particular kind of scoping where variables of a parent function are made available to inner functions as well. The scope of an inner function also includes the scope of a parent function. A polyfill is a way to provide new functionality available in modern JavaScript or a modern browser API to older browsers. A polyfill is a particular kind of shim.

A function that has no side effects does not modify external resources , and its output is only determined by the arguments. You could call this function 1M times, and given the same set of arguments, the output will always be the same. JavaScript with var and let declaration allows you to reassign a variable indefinitely. With const declarations you effectively declare an immutable value for strings, integers, booleans, and an object that cannot be reassigned but you can still modify it through its methods.

A shim is a little wrapper around a functionality, or API. You can consider it like a compatibility layer.You've successfully reported this review. JavaScript is now widely also used outside the browser. The syntax at a first glance is very simple, just use backticks instead of single or double quotes: The result is 2.

We basically have a function defined inside parentheses, and then we append to execute that function: Map and Set and their respective garbage collected WeakMap and WeakSet are the official implementations of two very popular data structures introduced later on. Default to const , and only use let if you reassign the variable.

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