The first thing we’ll study is the building blocks of code.… We can have as many statements in our code as we want.… Statements can be separated with a semicolon.… So, the code in the last example is treated as a single statement.… Such code will die with an error:
Our code must be as clean and easy to read as possible.… A good code style greatly assists in that.… See more about that in the chapter Code structure.… code more readable and easier to understand?”… Reading popular style guides will allow you to keep up to date with the latest ideas about code style
Make a pact with fellow ninjas of the team: if John starts “showing” functions with display... in his… A person with weak intuition would have to analyze the code line-by-line and track the changes through… every code branch.… You kill two rabbits with one shot.… A true ninja coder will make them not obvious from the code as well.
A code editor is the place where programmers spend most of their time.… There are two main types of code editors: IDEs and lightweight editors.… IDE.The term IDE (Integrated Development Environment) refers to a powerful editor with many features… For Windows, there’s also “Visual Studio”, not to be confused with “Visual Studio Code”.… The author’s personal opinion:
I’d use Visual Studio Code if I develop mostly frontend.
The built-in eval function allows to execute a string of code.… That negatively affects code compression ratio.… If eval’ed code doesn’t use outer variables, please call eval as window.eval(...):
This way the code… Summary.A call to eval(code) runs the string of code and returns the result of the last statement.… Instead, to eval the code in the global scope, use window.eval(code).
Wrote some code, testing: f(1) works, but f(2) doesn’t work. We fix the code and now f(2) works.… Chai – the library with many assertions.… The <script> with the function to test, in our case – with the code for pow.… So let’s continue with the second variant.… At the end we have both the spec and the code.
In the example below we see the other situation with .catch.… There’s no code to handle it.… In practice, just like with regular unhandled errors in code, it means that something has gone terribly… The script dies with a message in the console.… A similar thing happens with unhandled promise rejections.
In the modern HTML standard there’s a section about Drag and Drop with special events such as dragstart… Then on mousemove move it by changing left/top with position:absolute.… It runs automatically and conflicts with ours.… with better positioning:
In action… We can use that code to check what element we’re “flying over” at any time.
They work great with iterables, allowing to create data streams with ease.… The variant with a generator is much more concise than the original iterable code of range, and keeps… character codes 48…57),
followed by uppercase alphabet letters A..Z (character codes 65…90)
followed… The result is returned to the outer code.… The current line of the calling code is the line with generator.throw, labelled as (2).
JavaScript allows us to insert a character into a string by specifying its hexadecimal Unicode code with… whose Unicode code is XXXX.… Characters with Unicode values greater than U+FFFF can also be represented with this notation, but in… Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval… For instance, if we append a character “dot below” (code \u0323), then we’ll have “S with dots above
All previous declarations required us, programmers, to write the function code in the script.… Our new function needs to interact with the main script.… Minifiers are smart, they analyze the code structure, so they don’t break anything.… If new Function had access to outer variables, it would have problems with minifiers.… Besides, such code would be architecturally bad and prone to errors.
It gives a more descriptive way to compare a value with multiple variants.… If no case is matched then the default code is executed (if it exists).… b + 1 in case, and the corresponding code is executed.… Grouping of “case”.Several variants of case which share the same code can be grouped.… So we’ve got a dead code in case 3! The default variant will execute.
That’s not how we write new code.… “var” has no block scope.Variables, declared with var, are either function-scoped or global-scoped.… “var” tolerates redeclarations.If we declare the same variable with let twice in the same scope, that… So the code executes right away and has its own private variables.… Let’s note again: nowadays there’s no reason to write such code.
code.… They’ll ensure that the code works.… For example, later when you’re familiar with JavaScript, you can setup a code build system based on webpack… with the babel-loader plugin.… Most tutorial demos work with any modern browser though.
As we know from the chapter Code structure, comments can be single-line: starting with // and multiline… We normally use them to describe how and why the code works.… Recipe: factor out functions.Sometimes it’s beneficial to replace a code piece with a function, like… Such code is called self-descriptive.… And also the code structure is better when split.
External scripts.If we have a lot of JavaScript code, we can put it into a separate file.… A single <script> tag can’t have both the src attribute and code inside.… code.… We can use a <script> tag to add JavaScript code to a page.… There is much more to learn about browser scripts and their interaction with the webpage.
They allow the code to be called many times without repetition.… called with two arguments: from and "Hello"”.… For example, the jQuery framework defines a function with $.… Sometimes people refer to such code as self-describing.… They structure the code and make it readable.
Code blocks.If a variable is declared inside a code block {...}, it’s only visible inside that block.… What’s going on with the variables here?… A reference to the outer lexical environment, the one associated with the outer code.… Working with variables is actually working with the properties of that object.… old code).
Works with any iterable on the right-side
…Actually, we can use it with any iterable, not… Object destructuring.The destructuring assignment also works with objects.… We can use the rest pattern, just like we did with arrays.… flow (not inside another expression) as a code block.… And becomes unreadable when we deal with more parameters.
Destructuring comes to the rescue!
Even when a Promise is immediately resolved, the code on the lines below .then/.catch/.finally will still… That’s why “code finished” in the example above shows first.… That is, it first gets queued, then executed when the current code is complete and previously queued… How can we make code finished appear after promise done?… So .then/catch/finally handlers are always called after the current code is finished.
“KeyZ” and other key codes
Every key has the code that depends on its location on the keyboard… Key codes described in the UI Events code specification.… If we check event.code == 'KeyZ' in our code, then for people with German layout such test will pass… Luckily, that happens only with several codes, e.g. keyA, keyQ, keyZ (as we’ve seen), and doesn’t happen… with special keys such as Shift.
We already know how to work with them.… functions that execute on getting and setting a value, but look like regular properties to an external code… For instance, to create an accessor fullName with defineProperty, we can pass a descriptor with get and… Technically, external code is able to access the name directly by using user._name.… the old code that still uses age property?
How can we code it well?
Promises provide a couple of recipes to do that.… So in the code above all alert show the same: 1.… Please note that the code is still “flat” — it grows down, not to the right.… So we have the same problem as with callbacks.… The promise resolves with a response object when the remote server responds with headers, but before
a numeric code and a textual reason.… closure (used if no code supplied),
1006 – no way to set such code manually, indicates that the connection… WebSocket codes are somewhat like HTTP codes, but different.… In particular, codes lower than 1000 are reserved, there’ll be an error if we try to set such a code.… Here’s the code:
Server-side code is
That’s because the engine can’t understand the code.… So, try...catch can only handle errors that occur in valid code.… Should we be satisfied with that? Of course not!… That’s wrong and also makes the code more difficult to debug.… The try...catch construct may have one more code clause: finally.
Then your code will work well with both touch and mouse devices.… Replacing mouse<event> with pointer<event> in our code
We can replace mouse<… ;event> events with pointer<event> in our code and expect things to continue working fine with… of code.… We can replace mouse with pointer in event names and expect our code to continue working for mouse, with
Comparisons.Comparisons, such as <, > work with bigints and numbers just fine:… To emulate such behavior, a polyfill would need to analyze the code and replace all such operators with… In other words, this approach suggests that we write code in JSBI instead of native bigints.… But JSBI works with numbers as with bigints internally, emulates them closely following the specification… , so the code will be “bigint-ready”.
As user.address is undefined, an attempt to get user.address.street fails with an error.… As you can see, the "user.address" appears twice in the code.… That’s just awful, one may even have problems understanding such code.… As you can see, property names are still duplicated in the code.… E.g. in the code above, user.address appears three times.
That’s why the optional chaining ?.
If you’re not familiar with these methods, and their usage in the examples is confusing, you may want… Pyramid of Doom.At first glance, it looks like a viable approach to asynchronous coding.… So this way of coding isn’t very good.… It works, but the code looks like a torn apart spreadsheet.… That’s inconvenient, especially if the reader is not familiar with the code and doesn’t know where to
A “consuming code” that wants the result of the “producing code” once it’s ready.… A promise is a special JavaScript object that links the “producing code” and the “consuming code” together… But it’s fine to begin with.… Our code is only inside the executor.… Here’s an example of a promise constructor and a simple executor function with “producing code” that
For example, here’s a <div> with a contentEditable attribute.… If you run this code, it starts observing the element below and highlighting any code snippets that appear… Please run the previous code (above, observes that element), and then the code below.… A demo-element with id="highlight-demo", run the code above to observe it.… We can use it to track changes introduced by other parts of our code, as well as to integrate with third-party
We could implement the same kind of event listening in our code on our own, without the AbortController… But what’s valuable is that fetch knows how to work with the AbortController object.… Using with fetch.To be able to cancel fetch, pass the signal property of an AbortController as a fetch… When a fetch is aborted, its promise rejects with an error AbortError, so we should handle it, e.g. in… We can use AbortController in our code.
Before writing more complex code, let’s talk about debugging.… It also allows to trace the code step by step to see what exactly is going on.… The Code Editor pane shows the source code.… The “Step into” goes into their code, waiting for them if necessary.… Continue to here
Right click on a line of code opens the context menu with a great option
An object can be created with figure brackets {…} with an optional list of properties.… We can imagine an object as a cabinet with signed files.… So the in operator is an exotic guest in the code.… As an example, let’s consider an object with the phone codes:… the phone codes, we can “cheat” by making the codes non-integer.
JavaScript gives exceptional flexibility when dealing with functions.… By separating caching from the main function code we also keep the main code simpler.… Till now it was working only with single-argument functions.… That’s why it also works with this=arguments.… And all this without changing its code!
That is: each character has a corresponding numeric code.… The characters are compared by their numeric code.… The greater code means that the character is greater.… The code for a (97) is greater than the code for Z (90).… Here, its code is greater than anything from a to z.
Code structure.Statements are delimited with a semicolon:… Semicolons are not required after code blocks {...} and syntax constructs with them like loops:… More in: Code structure.… Characters $ and _ are normal, on par with letters.… with the page until they answer.
In JavaScript, a function is a value, so we can deal with it as a value.… The code above shows its string representation, which is the source code.… So we can work with it like with other kinds of values.… First, the syntax: how to differentiate between them in the code.… They can be assigned, copied or declared in any place of the code.
Another code may listen for the events and observe what’s happening with the menu.… If the event was created with the bubbles flag, then it bubbles.… Technically CustomEvent is the same as Event, with one exception.… And the code that dispatched it knows that it shouldn’t continue.… Summary.To generate an event from code, we first need to create an event object.
The fetch() method is modern and versatile, so we’ll start with it.… First, the promise, returned by fetch, resolves with an object of the built-in Response class as soon… as the server responds with headers.… if the HTTP status code is 200-299.… Sending an image.We can also submit binary data with fetch using Blob or BufferSource objects.
The code:
Such a code doesn’t care… And there’s an object with methods save, load, search… How to match them?… We don’t need to write the code to assign a handler to each button.… There can be as many attributes with data-counter as we want.… A click on an element with the attribute data-toggle-id will show/hide the element with the given id:
For example, outputting goods from a list one after another or just running the same code for each number… Loops are a way to repeat the same code multiple times.… Surely, we can just wrap the code in an if block instead of using continue.… In this case, code execution jumps to the next iteration of the labeled loop.… Such a loop, just like any other, can be stopped with the break directive.
Code is prone to errors. You will quite likely make errors… Oh, what am I talking about?… Other browsers also provide developer tools, sometimes with special features, but are usually playing… There’s an error in the JavaScript code on it.… This way one can enter long fragments of JavaScript code.… They can be opened with F12 for most browsers on Windows.
In object-oriented programming, a class is an extensible program-code-template for creating objects,… As we already know from the chapter Constructor, operator "new", new function can help with… The constructor runs with the given argument and assigns it to this.name.… The notation here is not to be confused with object literals.… All code inside the class construct is automatically in strict mode.
If you run the code below, the engine will “hang” for some time.… We can do that by wrapping the code in zero delay setTimeout.… Microtasks come solely from our code.… or new events handled, we can schedule it with queueMicrotask.… That’s a way to run code in another, parallel thread.
HttpError objects may have a statusCode property with a value like 404 or 403 or 500.… Here’s the code with MyError and other custom error classes, simplified:… The code which calls readUser should handle these errors.… So the outer code checks instanceof ReadError and that’s it.… It also works with inheritance.
Function or a string of code to execute.… Let’s compare two code fragments.… Let’s demonstrate what it means with the example below.… The setTimeout call in it re-schedules itself with zero delay.… zero-delay, and afterwards with 4+ ms delay.
Handlers are a way to run JavaScript code in case of user actions.… HTML-attribute.A handler can be set in HTML with an attribute named on<event>.… the same code, but that doesn’t matter, as it’s a different function object.… Only with addEventListener.… Also can’t write lots of code in there.
There’s a special syntax to work with promises in a more comfortable fashion, called “async/await”.… Async functions.Let’s start with the async keyword.… For instance, this function returns a resolved promise with the result of 1; let’s test it:… The function execution “pauses” at the line (*) and resumes when the promise settles, with… So the code above shows “done!” in one second.
Most of the time, a JavaScript application needs to work with information.… The statement below creates (in other words: declares) a variable with the name “message”:… When reading the code, COLOR_ORANGE is much more meaningful than #FF7F00.… In other words, capital-named constants are only used as aliases for “hard-coded” values.… Using different variables for different values can even help the engine optimize your code.
Only first 50 results are shown.