JS Closures an der LMU München

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What is a closure?

It is function + lexical environment

In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is called, at function creation time.

Closure is when a function can remember and access its lexical scope even when it's invoked outside its lexical scope. (e.g. when it is passed around as a value to a variable).

The inner function will have access to the variables in the outer function scope, even after the outer function has returned.

var add = (function () {

var counter = 0;

return function () {counter += 1; return counter}

})();

add();

// the counter is now 3

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What is the use of closures?

1. It can be used to make global variables local/private to a function. The Closure then has the function of getter and setter.

2. When leaving out the function, you can define a singelton pattern where the variable is only declared once.

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What is the use of bind()?

The bind() method creates a new function from an existing function, change the function's this context and provides any arguments you want the new function to be called with

e.g use bind() when a function belongs to an object

var module = {

x: 42,

getX: function() {

return this.x;

}

var unboundGetX = module.getX;

console.log(unboundGetX()); // The function gets invoked at the global scope

// expected output: undefined

var boundGetX = unboundGetX.bind(module);

console.log(boundGetX());

// expected output: 42

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What is hoisting?

Basically, when Javascript compiles all of your code, all variable declarations using var are hoisted/lifted to the top of their functional/local scope (if declared inside a function) or to the top of their global scope (if declared outside of a function) regardless of where the actual declaration has been made. This is what we mean by “hoisting”.

Function declarations are also hoisted, but these go to the very top, so will sit above all of the variable declarations.

However, a key thing to note is that the only thing that gets moved to the top is the variable declarations , not the actual value given to the variable. (so it returns undefined)

what we have to remember though is that the concept of hoisting is not a literal process (ie, the declarations themselves do not move to the top of the file — it is simply a process of the JavaScript compiler reading them first in order to create space in memory for them).

All identifiers are registered to their respective scopes during compile time. Moreover, every identifier is created at the beginning of the scope it belongs to, every time that scope is entered. The term for registering a variable at the top of its enclosing scope, even though its declaration may appear further down in the scope, is called hoisting.

let and const will throw reference errors. This is because although let and const are hoisted, they are not initialised until the code executes the initialising line. The time between a let or const being hoisted and initialised is commonly know as the temporal dead zone.

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What happens when you assign a variable that has not yet been declared?

If you try to set a variable that hasn't been declared, you'll either end up creating a variable in the top-level global scope (bad!) or getting an error, depending on "strict mode" (see "Strict Mode").

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What is the return value of: typeof [1,2,3] === typeof {val: 'a', val: 'b', val: 'c'};

Arrays are special objects, so the typeof of both expressions are objects and the return value is true. Run both expressions in your console to see that the return value of typeof [1,2,3] and typeof {1, 2, 3} is object.

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What is Polyfilling?

The word "polyfill" is an invented term used to refer to taking the definition of a newer feature and producing a piece of code that's equivalent to the behavior, but is able to run in older JS environments. For example, ES6 defines a utility called Number.isNaN(..) to provide an accurate non-buggy check for NaN values, deprecating the original isNaN(..) utility. But it's easy to polyfill that utility so that you can start using it in your code regardless of whether the end user is in an ES6 browser or not.

If the forwards-compatibility issue is not related to new syntax, but rather to a missing API method that was only recently added, the most common solution is to provide a definition for that missing API method that stands in and acts as if the older environment had already had it natively defined. This pattern is called a polyfill (aka "shim").

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What is Transpiling?

Transpiling is using a tool that converts your newer code into older code equivalents. The term combines the terms for transforming + compiling. Essentially, your source code is authored in the new syntax form, but what you deploy to the browser is the transpiled code in old syntax form. You typically insert the transpiler into your build process, similar to your code linter or your minifier.

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What gives a program state?

A program is described as stateful if it is designed to remember preceding events or user interactions. The remembered information is called the state. Without state, a program could perform some tasks, but they would be extremely limited in functionality.

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In a traditional compiled-language process, what is the correct order of the three main steps a program will undergo before executing?

Tokenizing/Lexing is the process of breaking up a string of characters into meaningful chunks called tokens. Parsing is the act of taking a stream of tokens and turning it into a tree (AST) of nested elements which represent the structure of the program. Code-Generation is the process of taking the tree and turning it into executable code.

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What is the responsibility of Scope?

The Scope is responsible for maintaing a look-up list of all declared identifiers and enforcing a strict set of rules as to how these are accessible to currently executing code.

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What happens during variable assignment?

Two distinct actions are taken for a variable assignment: First, Compiler declares a variable (if not previously declared in the current scope), and second, when executing, the Engine looks up the variable in Scope and assigns to it, if found.

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JS Closures

What is a closure?

It is function + lexical environment

In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is called, at function creation time.

Closure is when a function can remember and access its lexical scope even when it's invoked outside its lexical scope. (e.g. when it is passed around as a value to a variable).

The inner function will have access to the variables in the outer function scope, even after the outer function has returned.

var add = (function () {

var counter = 0;

return function () {counter += 1; return counter}

})();

add();

// the counter is now 3

JS Closures

What is the use of closures?

1. It can be used to make global variables local/private to a function. The Closure then has the function of getter and setter.

2. When leaving out the function, you can define a singelton pattern where the variable is only declared once.

JS Closures

What is the use of bind()?

The bind() method creates a new function from an existing function, change the function's this context and provides any arguments you want the new function to be called with

e.g use bind() when a function belongs to an object

var module = {

x: 42,

getX: function() {

return this.x;

}

var unboundGetX = module.getX;

console.log(unboundGetX()); // The function gets invoked at the global scope

// expected output: undefined

var boundGetX = unboundGetX.bind(module);

console.log(boundGetX());

// expected output: 42

JS Closures

What is hoisting?

Function declarations are also hoisted, but these go to the very top, so will sit above all of the variable declarations.

However, a key thing to note is that the only thing that gets moved to the top is the variable declarations , not the actual value given to the variable. (so it returns undefined)

JS Closures

What happens when you assign a variable that has not yet been declared?

JS Closures

What is the return value of: typeof [1,2,3] === typeof {val: 'a', val: 'b', val: 'c'};

JS Closures

What is Polyfilling?

JS Closures

What is Transpiling?

JS Closures

What gives a program state?

JS Closures

In a traditional compiled-language process, what is the correct order of the three main steps a program will undergo before executing?

JS Closures

What is the responsibility of Scope?

The Scope is responsible for maintaing a look-up list of all declared identifiers and enforcing a strict set of rules as to how these are accessible to currently executing code.

JS Closures

What happens during variable assignment?

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Karteikarten und Zusammenfassungen für JS Closures an der LMU München

Lerne jetzt mit Karteikarten und Zusammenfassungen für den Kurs JS Closures an der LMU München.

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