That can be done using constructor functions and the "new" operator.… Constructor function.Constructor functions technically are regular functions.… Constructor functions or, briefly, constructors, are regular functions, but there’s a common agreement… Constructor functions should only be called using new.… We can use constructor functions to make multiple similar objects.
Remember, new objects can be created with a constructor function, like new F().… The only thing that worked reliably was a "prototype" property of the constructor function,… Default F.prototype, constructor property.Every function has the "prototype" property even… function.… The "prototype" property only has such a special effect when set on a constructor function,
In JavaScript, there’s a distinction between a constructor function of an inheriting class (so-called… “derived constructor”) and other functions.… There’s no own constructor in Rabbit, so Animal constructor is called.… There’s no constructor() in it.… When overriding a constructor:
We must call parent constructor as super() in Child constructor before
As we already know from the chapter Constructor, operator "new", new function can help with… In JavaScript, a class is a kind of function.… The function code is taken from the constructor method (assumed empty if we don’t write such method).… There are two approaches to fixing it, as discussed in the chapter Function binding:
Pass a wrapper-function… Bind the method to object, e.g. in the constructor.
Let’s revisit arrow functions.
Arrow functions are not just a “shorthand” for writing small stuff.… functions do not have this.… functions can’t be used as constructors.… Arrow functions VS bind
There’s a subtle difference between an arrow function => and a… regular function called with .bind(this):
.bind(this) creates a “bound version” of the function.
Our function readUser(json) will not only read JSON, but check (“validate”) the data.… JavaScript requires us to call super in the child constructor, so that’s obligatory.… The parent constructor sets the message property.… The human-readable message is generated by the constructor.… " line in the constructor.
All built-in constructor functions use it.… the short notation obj = {} is the same as obj = new Object(), where Object is a built-in object constructor… function, with its own prototype referencing a huge object with toString and other methods.… For instance, when we create an array [1, 2, 3], the default new Array() constructor is used internally… Even functions – they are objects of a built-in Function constructor, and their methods (call/apply and
Their internal implementation uses the object’s constructor property for that.… If it exists, it should return the constructor that JavaScript will use internally to create new entities… So the extended functionality is not passed any further.
The value of this in User.staticMethod() call is the class constructor… Usually, static methods are used to implement functions that belong to the class as a whole, but not… For instance, we have Article objects and need a function to compare them.… The first way can be implemented by the constructor.… from Animal function.
For example, it can be used for building a polymorphic function, the one that treats arguments differently… It also works with constructor… functions:… It’s funny, but the Class constructor itself does not participate in the check!… Here we used call as described in the chapter Decorators and forwarding, call/apply to execute the function
The right constructor allows to specify standard properties for that type of event.… Below you can see a #rabbit and hide() function that dispatches "hide" event on it, to let… The generic Event(name, options) constructor accepts an arbitrary event name and the options object with… Other constructors of native events like MouseEvent, KeyboardEvent and so on accept properties specific… For custom events we should use CustomEvent constructor.
First, there’s a constructor, similar to Blob:… The constructor:
The main methods:… Its reading methods read* do not generate events, but rather return a result, as regular functions do
Many functions may need that result. These are the “fans”.… The function passed to new Promise is called the executor.… Here’s an example of a promise constructor and a simple executor function with “producing code” that… We’ve got the loadScript function for loading a script from the previous chapter.… The new function loadScript will not require a callback.
To catch an exception inside a scheduled function, try...catch must be inside that function:… For instance, we want to measure the time that a Fibonacci numbers function fib(n) takes.… But what if there’s an error during the function call?… In other words, the function may finish with return or throw, that doesn’t matter.… That JS script sets a custom window.onerror function.
Rendering in connectedCallback, not in constructor
In the example above, element content… Why not in the constructor?
The reason is simple: when constructor is called, it’s yet too early.… And if we’re making a special kind of button, why not reuse the existing <button> functionality
There are other kinds of objects in JavaScript: functions, for example, are objects.… One of the best things about objects is that we can store a function as one of its properties.… In order for that to work, a special “object wrapper” that provides the extra functionality is created… Constructors String/Number/Boolean are for internal use only
Some languages like Java allow… On the other hand, using the same functions
To create a weak reference, you need to use the WeakRef constructor, passing in the target object (the… that takes another function, fetchImg, as an argument.… In this example, we can neglect a detailed description of the fetchImg function, since it can be any… Return an anonymous function that takes the image name as an argument.… Get the DOM-element of the logs display window using the new WeakRef() constructor.
The prototype property of a constructor function has worked since very ancient times.… Object.setPrototypeOf and Object.getPrototypeOf were added to the standard, to perform the same functionality… Why was __proto__ replaced by the functions getPrototypeOf/setPrototypeOf?
Function Declaration.To create a function we can use a function declaration.… Local variables.A variable declared inside a function is only visible inside that function.… Then the function uses them.… Function naming:
A name should clearly describe what the function does.… A function is an action, so function names are usually verbal.
It provides the core tree functionality: parentNode, nextSibling, childNodes and so on (they are getters… But there are other classes that inherit from it (and so inherit the Node functionality).… To see the DOM node class name, we can recall that an object usually has the constructor property.… It references the class constructor, and constructor.name is its name:
Function is a value.Let’s reiterate: no matter how the function is created, a function is a value.… Function to run if the answer is “Yes”
no
Function to run if the answer is “No”
The function should… Function Expression vs Function Declaration.Let’s formulate the key differences between Function Declarations… creates the functions.… f = function(…) {…};.
That’s because setTimeout got the function user.sayHi, separately from the object.… That’s called partial function application – we create a new function by fixing some parameters of the… For instance, we have a function send(from, to, text).… Fortunately, a function partial for binding only arguments can be easily implemented.… When we fix some arguments of an existing function, the resulting (less universal) function is called
What type is a function?
In JavaScript, functions are objects.… handler functions to call.… Named Function Expression.Named Function Expression, or NFE, is a term for Function Expressions that… is only available for Function Expressions, not for Function Declarations.… They create a “main” function and attach many other “helper” functions to it.
But new Function allows to turn any string into a function.… But when a function is created using new Function, its [[Environment]] is set to reference not the current… Imagine that we must create a function from a string.… Our new function needs to interact with the main script.… To pass something to a function, created as new Function, we should use its arguments.
There’s another very simple and concise syntax for creating functions, that’s often better than Function… can be used in the same way as Function Expressions.… Arrow functions have other interesting features.… later in the chapter Arrow functions revisited.… For now, we can already use arrow functions for one-line actions and callbacks.
The constructor syntax is:
blobParts… From Blob to ArrayBuffer.The Blob constructor allows to create a blob from almost anything, including… Blob and low-level binary data types:
We can make a Blob from a typed array using new Blob(...) constructor
Please note, there’s no constructor called TypedArray, it’s just a common “umbrella” term to represent… A typed array constructor (be it Int8Array or Float64Array, doesn’t matter) behaves differently depending… For typed arrays, the constructor dictates what the format is.
Currying is an advanced technique of working with functions.… Currying is a transformation of functions that translates a function from callable as f(a, b, c) into… Currying doesn’t call a function. It just transforms it.… The result of curry(func) is a wrapper function(a).… Fixed-length functions only
The currying requires the function to have a fixed number of
is (assuming that the definition is not in the nested function).… The Function Expression is wrapped with parenthesis (function {...}), because when JavaScript engine… encounters "function" in the main code, it understands it as the start of a Function Declaration… hence it’s a Function Expression: it needs no name and can be called immediately.… , or global, if declared outside function.
var declarations are processed at function start (script start
A function can be created at any moment, passed as an argument to another function, and then called from… Nested functions.A function is called “nested” when it is created inside another function.… Function Declarations.A function is also a value, like a variable.… That’s why we can use a function, declared as Function Declaration, even before the declaration itself… Expressions where we assign a function to a variable, such as let say = function(name)....
timing(timeFraction)
Timing function, like CSS-property transition-timing-function that gets the fraction… This is that function that actually draws out the animation.… and any drawing function here.… The timing function is not limited by Bezier curves.… JavaScript animations can use any timing function.
Arrow functions have no “this”.Arrow functions are special: they don’t have their “own” this.… If we reference this from such a function, it’s taken from the outer “normal” function.… Later in the chapter Arrow functions revisited we’ll go more deeply into arrow functions.
Summary.… When a function is declared, it may use this, but that this has no value until the function is called… A function can be copied between objects.
It’s the conversion of a function that accepts a callback into a function that returns a promise.… is a wrapper around the original loadScript function.… We’ll call it promisify(f): it accepts a to-promisify function f and returns a wrapper function.… In Node.js, there’s a built-in util.promisify function for that.… So promisification is only meant for functions that call the callback once.
But instead of adding that functionality into slow() we’ll create a wrapper function, that adds caching… We can apply it to another function.… There’s a special built-in function method func.call(context, …args) that allows to call a function explicitly… Decorators and function properties.It is generally safe to replace a function or a method with a decorated… a special Proxy object to wrap a function.
For instance, one such function is the setTimeout function.… It declares new functions, and we want to run them.… As of now, the loadScript function doesn’t provide a way to track the load completion.… But we’d like to know when it happens, to use new functions and variables from that script.… A function that does something asynchronously should provide a callback argument where we put the function
For instance, we’re creating a function f.… () { ... })
What functionality we’re describing?… While the functionality is not complete, errors are displayed.… Sinon – a library to spy over functions, emulate built-in functions and more, we’ll need it much later… Extending the spec.The basic functionality of pow is complete.
It can be placed before a function, like this:… The word “async” before a function means one simple thing: a function always returns a promise.… There’s another keyword, await, that works only inside async functions, and it’s pretty cool.… Can’t use await in regular functions
If we try to use await in a non-async function, there… As stated earlier, await only works inside an async function.
We may decide to execute a function not right now, but at a certain time later.… There are two methods for it:
setTimeout allows us to run a function once after the interval of time… Usually, that’s a function.… They may take much more memory than the function itself.… So the function is scheduled to run “right after” the current script.
Regular functions return only one, single value (or nothing).… generator function”.… function* f(…) or function *f(…)?
Both syntaxes are correct.… In a regular function, to combine results from multiple other functions, we call them, store the results… Generators are created by generator functions function* f(…) {…}.
Many JavaScript built-in functions support an arbitrary number of arguments.… And also, how to pass arrays to such functions as parameters.… Rest parameters ....A function can be called with any number of arguments, no matter how it is defined… The rest of the parameters can be included in the function definition by using three dots ... followed… an arrow function, it takes them from the outer “normal” function.
Functions.We covered three ways to create a function in JavaScript:
Function Declaration: the function… Arrow functions:
Functions… Such variables are only visible inside the function.… Parameters can have default values: function sum(a = 1, b = 2) {...}.… Details: see Functions, Arrow functions, the basics.
The global object provides variables and functions that are available anywhere.… In a browser, global functions… declarations have the same effect (statements with function keyword in the main code flow, not function… The code design where a function gets “input” variables and produces certain “outcome” is clearer, less… In-browser, unless we’re using modules, global functions and variables declared with var become a property
The built-in eval function allows to execute a string of code.… A string of code may be long, contain line breaks, function… So functions and variables, declared inside eval, are not visible outside:… construct is explained in the chapter The "new Function" syntax.… It creates a function from a string, also in the global scope. So it can’t see local variables.
If we add parentheses, then sayThanks() becomes a function call.… So the last line actually takes the result of the function execution, that is undefined (as the function… To remove a handler we should pass exactly the same function as was assigned.… – with the same code, but that doesn’t matter, as it’s a different function object.… DOM property: elem.onclick = function.
Local shows local function variables.… Global has global variables (out of any functions).… (not a built-in, like alert, but a function of our own).… first line, while “Step over” executes the nested function call invisibly to us, skipping the function… The execution is then paused immediately after that function call.
Recipe: factor out functions.Sometimes it’s beneficial to replace a code piece with a function, like… The function itself becomes the comment. Such code is called self-descriptive.… It’s clear what every function does, what it takes and what it returns.… Document function parameters and usage
There’s a special syntax JSDoc to document a function: usage,… Function usage.
Important solutions, especially when not immediately obvious.
When a function solves a task, in the process it can call many other functions.… For that we’ll look under the hood of functions.… When a function makes a nested call, the following happens:
The current function is paused.… When a function calls itself, that’s called a recursion step.… The basis of recursion is function arguments that make the task so simple that the function does not
For instance, consider function prefixes.… For instance, the function printPage(page) will use a printer.… And the function printText(text) will put the text on-screen.… In a function try to use only variables passed as parameters.… There are functions that look like they don’t change anything.
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