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For instance, one such function is the setTimeout function.… Let’s add a callback function as a second argument to loadScript that should execute when the script… Now if we want to call new functions from the script, we should write that in the callback:… A function that does something asynchronously should provide a callback argument where we put the function… So the single callback function is used both for reporting errors and passing back results.

It’s the conversion of a function that accepts a callback into a function that returns a promise.… Such transformations are often required in real-life, as many functions and libraries are callback-based… The function loads a script with the given src, and then calls callback(err) in case of an error, or… Here, promisify assumes that the original function expects a callback with exactly two arguments (err… So promisification is only meant for functions that call the callback once.

Function is a value.Let’s reiterate: no matter how the function is created, a function is a value.… Callback functions.Let’s look at more examples of passing functions as values and using function expressions… Function to run if the answer is “Yes” no Function to run if the answer is “No” The function should… The arguments showOk and showCancel of ask are called callback functions or just callbacks.… Function Expression vs Function Declaration.Let’s formulate the key differences between Function Declarations

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.

callbacks and with CSS animations.… timing(timeFraction) Timing function, like CSS-property transition-timing-function that gets the fraction… and any drawing function here.… The timing function is not limited by Bezier curves.… That built-in method allows to setup a callback function to run when the browser will be preparing a

that takes another function, fetchImg, as an argument.… Return an anonymous function that takes the image name as an argument.… Before we move on, let’s clarify the terminology: Cleanup callback (finalizer) – is a function that is… This mechanism allows registering an object to track and associate a cleanup callback with it.… , and then automatically invokes those callbacks when the objects are deleted from memory.

Many functions may need that result. These are the “fans”.… Its arguments resolve and reject are callbacks provided by JavaScript itself.… We’ve got the loadScript function for loading a script from the previous chapter.… The new function loadScript will not require a callback.… We must have a callback function at our disposal when calling loadScript(script, callback).

MutationObserver is a built-in object that observes a DOM element and fires a callback when it detects… First, we create an observer with a callback-function:… other options: attributeOldValue – if true, pass both the old and the new value of attribute to callback… Then after any changes, the callback is executed: changes are passed in the first argument as a list… Imagine the situation when you need to add a third-party script that contains useful functionality, but

When passing object methods as callbacks, for instance to setTimeout, there’s a known problem: “losing… 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).… When we fix some arguments of an existing function, the resulting (less universal) function is called

Let’s return to the problem mentioned in the chapter Introduction: callbacks: we have a sequence of asynchronous… So we have the same problem as with callbacks.… Sometimes it’s ok to write .then directly, because the nested function has access to the outer scope.… In the example above the most nested callback has access to all variables script1, script2, script3.… in line (*): if it has a callable method named then, then it calls that method providing native functions

The callback function passed in forEach has 3 arguments: a value, then the same value valueAgain, and… That’s for compatibility with Map where the callback passed forEach has three arguments.

We may decide to execute a function not right now, but at a certain time later.… Usually, that’s a function.… Garbage collection and setInterval/setTimeout callback When a function is passed in setInterval… They may take much more memory than the function itself.… So the function is scheduled to run “right after” the current script.

the data from http://another.com, such as the weather: First, in advance, we declare a global function… callback=gotWeather", using the name of our function as the callback URL-parameter.… When the remote script loads and executes, gotWeather runs, and, as it’s our function, we have the data

When the time is due for a scheduled setTimeout, the task is to run its callback. …and so on.… demonstrate this approach, for the sake of simplicity, instead of text-highlighting, let’s take a function… On one hand, that’s great, because our function may create many elements, add them one-by-one to the… Here’s the demo, the changes to i won’t show up until the function finishes, so we’ll see only the last… There’s also a special function queueMicrotask(func) that queues func for execution in the microtask

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.

There’s no built-in callback that triggers after nested elements are ready.… And if we’re making a special kind of button, why not reuse the existing <button> functionality

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.

Let’s revisit arrow functions. Arrow functions are not just a “shorthand” for writing small stuff.… functions do not have this.… 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.… The function simply doesn’t have this.

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.

Call canvas method .toBlob(callback, format, quality) that creates a Blob and runs callback with it when… If we prefer async/await instead of callbacks

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)....

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.

Constructor function.Constructor functions technically are regular functions.… Let’s note once again – technically, any function (except arrow functions, as they don’t have this) can… 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.

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, 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.

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.

objects, providing initial values for state (member variables) and implementations of behavior (member functions… As we already know from the chapter Constructor, operator "new", new function can help with… In JavaScript, a class is a kind of function.… binding functions in JavaScript have a dynamic this.… There are two approaches to fixing it, as discussed in the chapter Function binding: Pass a wrapper-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.

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… Yes, it exists in the default "prototype" for functions, but that’s all.… The "prototype" property only has such a special effect when set on a constructor function,

Please note that export before a class or a function does not make it a function expression… It’s still a function declaration, albeit exported.… Most JavaScript style guides don’t recommend semicolons after function and class declarations.… Modules that contain a library, pack of functions, like say.js above.… /…: export [default] class/function/variable ...

For example, Math.trunc(n) is a function that “cuts off” the decimal part of a number, e.g Math.trunc… As we’re talking about new functions, not syntax changes, there’s no need to transpile anything here.… We just need to declare the missing function.… A script that updates/adds new functions is called “polyfill”.… Scripts may add/modify any function, even built-in ones.

That’s typical, other arguments of this function are rarely used.… A comparison function may return any number Actually, a comparison function is only required… for the best Remember arrow functions?… As the function is applied, the result of the previous function call is passed to the next one as the… So the function result is 1.

The module path must be a primitive string, can’t be a function call.… Or, we could use let module = await import(modulePath) if inside an async function… Please note: Although import() looks like a function call, it’s a special syntax that just… It’s not a function.

So we can create new functionality on top of the existing.… Arrow functions have no super As was mentioned in the chapter Arrow functions revisited,… arrow functions do not have super.… If accessed, it’s taken from the outer function.… “derived constructor”) and other functions.

However, when we pass these to a function, we may not need all of it.… The function might only require certain elements or properties.… parameters, default values can be any expressions or even function calls.… Smart function parameters.There are times when a function has many parameters, most of which are optional… Imagine a function that creates a menu.

A module may contain a class or a library of functions for a specific purpose.… , call functions of one module from another one: export keyword labels variables and functions that… In other words, a module can provide a generic functionality that needs a setup.… Some functionality may not work yet.… When we use modules, each module implements the functionality and exports it.

To make user.hi() calls work, JavaScript uses a trick – the dot '.' returns not a function, but a value… The result of a property access user.hi is not a function, but a value of Reference Type.… like assignment hi = user.hi discards the reference type as a whole, takes the value of user.hi (a function… So, as the result, the value of this is only passed the right way if the function is called directly… For all other operations, the reference type automatically becomes the property value (a function in

-- ```js function pow(x, n) { let result = 1; for (let i = 0; i < n; i++) { result *= x; }… Even a single function can often be divided into logical blocks.… Function Placement.If you are writing several “helper” functions and the code that uses them, there are… three ways to organize the functions.… Mixed: a function is declared where it’s first used.

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.… So, Rabbit extends Animal creates two [[Prototype]] references: Rabbit function prototypally inherits… from Animal function.… Summary.Static methods are used for the functionality