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Let’s revisit arrow functions. Arrow functions are not just a “shorthand” for writing small stuff.… That’s where arrow functions come in handy.… Arrow functions have no “this”.As we remember from the chapter Object methods, "this", arrow… Arrow functions VS bind There’s a subtle difference between an arrow function => and a… Arrows have no “arguments”.Arrow functions also have no arguments variable.

It’s called “arrow functions”, because it looks like this:… Multiline arrow functions.The arrow functions that we’ve seen so far were very simple.… Arrow functions have other interesting features.… functions later in the chapter Arrow functions revisited.… For now, we can already use arrow functions for one-line actions and callbacks.

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.… Please note that arrow functions are special: they have no this.… When this is accessed inside an arrow function, it is taken from outside.

Many JavaScript built-in functions support an arbitrary number of arguments.… And also, how to pass arrays to such functions as parameters.… Arrow functions do not have "arguments" If we access the arguments object from… an arrow function, it takes them from the outer “normal” function.… functions don’t have their own this.

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.… “derived constructor”) and other functions.… Also: Arrow functions don’t have their own this or super, so they transparently fit into the surrounding

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.

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)....

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,… picture: On the picture, "prototype" is a horizontal arrow… 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.

functions:… Sometimes it’s ok to write .then directly, because the nested function has access to the outer scope.… in line (*): if it has a callable method named then, then it calls that method providing native functions… We’ll also use arrow functions for brevity:… Finally, we can split the code into reusable functions:

That can be done using constructor functions and the "new" operator.… 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.

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… functions 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.

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.

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.

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… – with the same code, but that doesn’t matter, as it’s a different function object.… That’s exactly the same as this, unless the handler is an arrow function, or its this is bound to something… DOM property: elem.onclick = function.

counted from the document top (now scrolled out). clientY – window-relative coordinate did change (the arrow… For instance, the function createMessageUnder(elem, html) below shows the message under elem:… The function getCoords(elem) will take window coordinates from elem.getBoundingClientRect() and add the… The modified createMessageUnder function:

We create primitives, objects, functions… All that takes memory.… For instance: The currently executing function, its local variables and parameters.… Other functions on the current chain of nested calls, their local variables and parameters.… Here the arrow… The family: Function marry “marries

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.

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.

For instance, to react on arrow keys Up and Down or hotkeys (including combinations of keys).… Let’s relax the filter a little bit by allowing arrow keys Left, Right and Delete, Backspace:… Now arrows… might still use the right values for e.key, e.code, e.keyCode… when pressing certain keys such as arrows

doesn’t trigger on keyboard input and other actions that do not involve value change, e.g. pressing arrow

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

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.

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.

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(…) {…}.

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

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.

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.

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.

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.

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

Many functions may need that result. These are the “fans”.… These functions are pre-defined by the JavaScript engine, so we don’t need to create them.… 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).

-- ```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

Wrapping functions: "apply".We can wrap a proxy around a function as well.… The wrapper function (*) performs the call after the timeout.… But a wrapper function does not forward property read/write operations or anything else.… It can wrap any kind of object, including classes and functions.… Calling a function (apply trap). The new operator (construct trap).

All built-in constructor functions use it.… notation obj = {} is the same as obj = new Object(), where Object is a built-in object constructor function… Other built-in prototypes.Other built-in objects such as Array, Date, Function… Even functions – they are objects of a built-in Function constructor, and their methods (call/apply and… Borrowing methods is flexible, it allows to mix functionalities from different objects if needed.

Namely: Function properties (methods). Symbolic keys and values.… function(key, value). space Amount of space to use for formatting Most of the time, JSON.stringify… Fortunately, we can use a function instead of an array as the replacer.… The function will be called for every (key, value) pair and should return the “replaced” value, which… Both methods support transformer functions for smart reading/writing.