What is JavaScript?.JavaScript was initially created to “make web pages alive”.… For instance, in-browser JavaScript is able to:
Add new HTML to the page, change the existing content… JavaScript’s abilities in the browser are limited to protect the user’s safety.… That said, JavaScript can be used to create servers, mobile applications, etc.… It is recommended to take a look at them, at least briefly, after mastering JavaScript.
JavaScript animations can handle things that CSS can’t.… There’s one more thing to keep in mind.… That built-in method allows to setup a callback function to run when the browser will be preparing a… Gets a time fraction from 0 to 1, returns the animation progress, usually from 0 to 1.
draw – the function… to draw the animation.
to subtle moments.… converts the operand to boolean type and returns the inverse value.… Later we’ll study more types of loops to deal with objects.… Functions.We covered three ways to create a function in JavaScript:
Function Declaration: the function… More to come.That was a brief list of JavaScript features. As of now we’ve studied only basics.
It also allows to trace the code step by step to see what exactly is going on.… -- https://github.com/ChromeDevTools/devtools-frontend/blob/master/front_end/Images/src/largeIcons.svg… first line, while “Step over” executes the nested function call invisibly to us, skipping the function… That’s similar to “Step”, but behaves differently in case of asynchronous function calls.… That’s handy when we want to move multiple steps forward to the line, but we’re too lazy to set a breakpoint
CSS animations make it possible to do simple animations without JavaScript at all.… JavaScript can be used to control CSS animations and make them even better, with little code.… #stripe to the left step-by-step.… We’re also able to use JavaScript for animations, the next chapter is devoted to that.… But in the next chapter we’ll do some JavaScript animations to cover more complex cases.
JavaScript doesn’t allow you to customize how operators work on objects.… In this chapter we’ll cover how an object converts to primitive and how to customize it.… Hints.How does JavaScript decide which conversion to apply?… To do the conversion, JavaScript tries to find and call three object methods:
Call obj[Symbol.toPrimitive… Let’s implement these methods to customize the conversion.
Handlers are a way to run JavaScript code in case of user actions.… An HTML-attribute is not a convenient place to write a lot of code, so we’d better create a JavaScript… to one event.… Please note that we need to explicitly setup the events to listen using addEventListener.… Methods: elem.addEventListener(event, handler[, phase]) to add, removeEventListener to remove.
10.… In practice, the browser provides ways to stop such loops, and in server-side JavaScript, we can kill… If you are new to loops, it could help to go back to the example and reproduce how it runs step-by-step… If we don’t want to do anything in the current iteration and would like to forward to the next one, we… A label is the only way for break/continue to escape a nested loop to go to an outer one.
JavaScript also did not have any special methods to perform network requests at that time.… For a long time JavaScript was unable to do such requests.… To grant JavaScript access to any other response header, the server must send the Access-Control-Expose-Headers… JavaScript only gets the response to the main request or an error if there’s no server permission.… same value as Origin
Access-Control-Allow-Credentials to true
Additionally, to grant JavaScript access
It is not convenient to use an object here, because it provides no methods to manage the order of elements… They allow you to add/remove elements, both to/from the beginning or the end.… Arrays are carefully tuned inside JavaScript engines to work with contiguous ordered data, please use… Move all elements to the left, renumber them from the index 1 to 0, from 2 to 1 and so on.… to move existing elements to the right, increasing their indexes.
In this chapter we’ll see the ways to fix it.… The task is quite typical – we want to pass an object method somewhere else (here – to the scheduler)… Then, inside a user object we may want to use a partial variant of it: sendTo(to, text) that sends from… :00")
Then gives it ...args – arguments given to the wrapper ("Hello")
So easy to do… For example, to setTimeout.
To create a custom element, we need to tell the browser several details about it: how to show it, what… to do when the element is added or removed to page, etc.… HTML parser connected the custom element <user-info>, and is going to proceed to its children,… If we’d like to pass information to custom element, we can use attributes.… There’s a polyfill https://github.com/webcomponents/polyfills/tree/master/packages/webcomponentsjs.
We’ll talk about let/const variables here
In JavaScript, there are 3 ways to declare a variable… It is easily possible to do this with JavaScript.… JavaScript engines also may optimize it, discard variables that are unused to save memory and perform… Step 3.… But in practice, JavaScript engines try to optimize that.
Memory management in JavaScript is performed automatically and invisibly to us.… Detailed examples to follow.… There’s no way to access it, no references to it.… JavaScript engines apply many optimizations to make it run faster and not introduce any delays into the… It would be wise to plan that as the next step after you’re familiar with the language.
Being very easy to implement, it’s also good enough in a lot of cases.… For example, once every 10 seconds.… That works, but there are downsides:
Messages are passed with a delay up to 10 seconds (between requests… That’s quite a load to handle, speaking performance-wise.… The flow:
A request is sent to the server.
More ways to write a number.Imagine we need to write 1 billion.… In JavaScript, we can shorten a number by appending the letter "e" to it and specifying the… By default, it’s 10.… In the middle cases 3.5 rounds up to 4, and -3.5 rounds up to -3.… So, division by powers 10 is guaranteed to work well in the decimal system, but division by 3 is not.
At the end you’ll find a good recipe to avoid “JavaScript quirks”-related issues.… In the first example above, the comparison 'Z' > 'A' gets to a result at the first step.… to numbers.… What to do if we’d like to differentiate 0 from false?… == analogous to !=.
The fetch method allows to track download progress.… To log the progress, we just need for every received fragment value to add its length to the counter.… Let’s explain that step-by-step… method to get the result.… To create a string, we need to interpret these bytes. The built-in TextDecoder does exactly that.
Let’s step away from the individual data structures and talk about the iterations over them.… These methods are generic, there is a common agreement to use them for data structures.… Remember, objects are a base of all complex structures in JavaScript.… (obj) to get an array of key/value pairs from obj.… Use array methods on that array, e.g. map, to transform these key/value pairs.
x to a natural power of n.… There are two ways to implement it.… For example, to calculate pow(2, 4) the recursive variant does these steps:
pow(2, 4) = 2 * pow(2, 3… ).To do a nested call, JavaScript remembers the current execution context in the execution context stack… , to move back easily.
Mind the units.Don’t forget to add CSS units to values.… For instance, we should not set elem.style.top to 10, but rather to 10px.… How to do it?… to break the privacy.… To see how to apply important and other rare stuff – there’s a list of methods at MDN.
The first thing to do is to locate quoted strings, and then we can replace them.… Then it advances: goes to the next positions in the source string and tries to find the first character… It tries to see if the rest of the subject string conforms to .+".… clearly understand the change, let’s trace the search step by step.… But to understand how regular expressions work and to build regular expressions, we don’t need to know
But we forgot to re-test f(1). That may lead to an error.
That’s very typical.… Now, before making the implementation, let’s use a few JavaScript libraries to run the tests, just to… We need to add more use cases to it.
Let’s add one more test to check that pow(3, 4) = 81.… To indicate a mathematical error, JavaScript functions usually return NaN.… Writing tests requires good JavaScript knowledge. But we’re just starting to learn it.
For that, we need to set deleteCount to 0:… JavaScript much later and uses the more up-to-date comparison algorithm internally.… To use our own sorting order, we need to supply a function as the argument of arr.sort().… To sort it, we need an ordering function that knows how to compare its elements.… return a positive number to say “greater” and a negative number to say “less”.
The regular {...} syntax allows us to create one object.… :
A new empty object is created and assigned to this.… The “capital letter first” is a common agreement, to make it clear that a function is to be run with… The constructor function may have parameters that define how to construct the object, and what to put… for sets and others that we plan to study.
to cancel the execution.… For instance, we need to write a service that sends a request to the server every 5 seconds asking for… data, but in case the server is overloaded, it should increase the interval to 10, 20, 40 seconds…
Here… For server-side JavaScript, that limitation does not exist, and there exist other ways to schedule an… Nested setTimeout calls are a more flexible alternative to setInterval, allowing us to set the time between
There are many JavaScript properties that allow us to read information about element width, height and… We often need them when moving or positioning elements in JavaScript.… offsetWidth/Height.Now let’s move on to the element itself.… We can use these properties to expand the element wide to its full width/height.… ClickMe123456789
Setting scrollTop to 0 or a big value, such as 1e9 will make the element scroll to the
Many functions are provided by JavaScript host environments that allow you to schedule asynchronous actions… But we’d like to know when it happens, to use new functions and variables from that script.… It’s difficult to read, and you probably noticed that one needs to eye-jump between pieces while reading… Also, the functions named step* are all of single use, they are created only to avoid the “pyramid of… We’d like to have something better.
Luckily, there are other ways to avoid such pyramids.
To highlight the code, it performs the analysis, creates many colored elements, adds them to the document… Periodic returns to the event loop between count executions provide just enough “air” for the JavaScript… engine to do something else, to react to other user actions.… Go to step 1.
To schedule a new macrotask:
Use zero delayed setTimeout(f).… That may be used to split a big calculation-heavy task into pieces, for the browser to be able to react
XMLHttpRequest is a built-in browser object that allows to make HTTP requests in JavaScript.… We need to support old browsers, and don’t want polyfills (e.g. to keep scripts tiny).… To do the request, we need 3 steps:
Create XMLHttpRequest:… And some of them like POST use body to send the data to the server.… URL search parameters
To add parameters to URL, like ?
The instanceof operator allows to check whether an object belongs to a certain class.… is:
It returns true if obj belongs to… Please note that arr also belongs to… that case, the standard logic is used: obj instanceOf Class checks whether Class.prototype is equal to… a string rather than just to check.
After some practice it becomes obvious how to place points to get the needed curve.… The parameter t moves from 0 to 1.… As t runs from 0 to 1, every value of t adds a point to the curve.… to t from the beginning.… Then for each t from 0 to 1, we take a point on each segment on the distance proportional to t and connect
DOM modification is the key to creating “live” pages.… Here’s an example of using these methods to add items to a list and the text before/after it:… The first parameter is a code word, specifying where to insert relative to elem.… move an element to another place – there’s no need to remove it from the old one.… We can use JavaScript to create a full-fledged webpage and write it.
To perform the update to version 2, all connections to version 1 must be closed, including the one in… functions: from 1 to 2, from 2 to 3, from 3 to 4 etc.… by step, for every intermediate version (2 to 3, then 3 to 4).… Let’s say we want to search by price.
First, we need to create an index.… Then getAll will fail to get all records as an array.
What to do?
These comments hide JavaScript code from old browsers that didn’t know how to process the <script&… Here, /path/to/script.js is an absolute path to the script from the site root.… We can use a <script> tag to add JavaScript code to a page.… But let’s keep in mind that this part of the tutorial is devoted to the JavaScript language, so we shouldn… We’ll be using the browser as a way to run JavaScript, which is very convenient for online reading, but
The JavaScript language steadily evolves.… to the list at https://tc39.github.io/ecma262/ and then progress to the specification.… Teams behind JavaScript engines have their own ideas about what to implement first.… Summary.In this chapter we’d like to motivate you to study modern and even “bleeding-edge” language features… Just don’t forget to use a transpiler (if using modern syntax or operators) and polyfills (to add functions
There are eight basic data types in JavaScript.… To be really precise, the “number” type can store larger integers (up to 1.7976931348623157 * 10308),… BigInt type was recently added to the language to represent integers of arbitrary length.… It’s useful when we want to process values of different types differently or just want to do a quick… Functions belong to the object type.
To start, we’ll learn how to open them, look at errors, and run JavaScript commands.… It marks a “command line” where we can type JavaScript commands. Press Enter to run them.… Once you know how to use one of these tools (you can start with Chrome), you can easily switch to another… We need to enable the “Develop menu” first.
Open Preferences and go to the “Advanced” pane.… In the next section, we’ll get down to JavaScript.
JavaScript allows us to work with primitives (strings, numbers, etc.) as if they were objects.… They also provide methods to call as such.… It would be great to access them using methods.… They convert a value to the corresponding type: to a string, a number, or a boolean (primitive).… Formally, these methods work via temporary objects, but JavaScript engines are well tuned to optimize
In programming, we often want to take something and extend it.… Then, when alert tries to read property rabbit.eats (**), it’s not in rabbit, so JavaScript follows the… JavaScript will throw an error if we try to assign __proto__ in a circle.… __proto__ to access it (a historical getter/setter, there are other ways, to be covered soon).… If we want to read a property of obj or call a method, and it doesn’t exist, then JavaScript tries to
The JavaScript language was initially created for web browsers.… In practice though, the CSSOM is rarely required, because we rarely need to modify CSS rules from JavaScript… The location object allows us to read the current URL and can redirect the browser to a new one.… Please note these links, as there’s so much to learn that it’s impossible to cover everything and remember… To find something, it’s often convenient to use an internet search “WHATWG [term]” or “MDN [term]”, e.g
Initially it was made for JavaScript, but many other languages have libraries to handle it as well.… So it’s easy to use JSON for data exchange when the client uses JavaScript and the server is written… JavaScript provides methods:
JSON.stringify to convert objects into JSON.… deserialize it, to turn back into JavaScript object.… JavaScript provides methods JSON.stringify to serialize into JSON and JSON.parse to read from JSON.
It aims to help you gradually learn the language.… But being that formalized, it’s difficult to understand at first.… You can find it at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference.… Although, it’s often best to use an internet search instead.… q=MDN+parseInt to search for the parseInt function.
Line Length.No one likes to read a long horizontal line of code. It’s best practice to split them.… the code vulnerable to errors.… Otherwise, it’s best to use semicolons to avoid possible pitfalls.… Style Guides.A style guide contains general rules about “how to write” code, e.g. which quotes to use… Reading popular style guides will allow you to keep up to date with the latest ideas about code style
For instance, we can use it to store creation/modification times, to measure time, or just to print out… That may lead to wrong results.… engines start applying advanced optimizations only to “hot code” that executes many times (no need to… So if you seriously want to understand performance, then please study how the JavaScript engine works… JavaScript itself does not have a way to measure time in microseconds (1 millionth of a second), but
Most of the time we can replace let by var or vice-versa and expect things to work:… On the other hand, it’s important to understand differences when migrating old scripts from var to let… , to avoid odd errors.… There exist other ways besides parentheses to tell JavaScript that we mean a Function Expression:… Let’s note again: nowadays there’s no reason to write such code.
As we already know, a function in JavaScript is a value.
Every value in JavaScript has a type.… In JavaScript, functions are objects.… handler functions to call.… Sometimes, when we need a reliable internal name, it’s the reason to rewrite a Function Declaration to… Usually taken from the function definition, but if there’s none, JavaScript tries to guess it from the
sending it to the server or to abort the submission and process it in JavaScript.… The method form.submit() allows to initiate form sending from JavaScript.… We can use it to dynamically create and send our own forms to server.… Event: submit.There are two main ways to submit a form:
The first – to click <input type="submit… Both actions lead to submit event on the form.
The built-in eval function allows to execute a string of code.… Right now, there’s almost no reason to use eval.… Code minifiers (tools used before JS gets to production, to compress it) rename local variables into… shorter ones (like a, b etc) to make the code smaller.… There are two ways how to be totally safe from such problems.
But there are situations where JavaScript “fails” to assume a semicolon where it is really needed.… Errors which occur in such cases are quite hard to find and fix.… to see it).… We need to put a semicolon after alert for the code to work correctly.… But it’s safer – especially for a beginner – to use them.
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