The two of the following result in different things although they look like the same thing.
1
const addBlogPost = dispatch => {
return () => {
dispatch({type: 'add_blogpost'});
}
};
2
const addBlogPost = dispatch => dispatch({type: 'add_blogpost'});
Could anyone point out how are they different?
You can use this site to compile es6 arrow functions to vanilla JS to easily see the difference.
The first one compiles to this
var addBlogPost = function addBlogPost(dispatch) {
return function () {
dispatch({
type: 'add_blogpost'
});
};
};
While the second compiles to this
var addBlogPost = function addBlogPost(dispatch) {
return dispatch({
type: 'add_blogpost'
});
};
The first returns a function that has a dispatch while the second one returns a dispatch directly.
The result will always be the same as both functions are retuning the same thing.
The only difference is:
In the first function, you're returning a function which returns an object that returns dispatch function.
In the second function, you're returning your dispatch function directly.
Is there a difference between these two function declarations in an es6 class? Is the scope this same (this)? Is one way preferred?
class Node {
// function declaration 1
test () {
}
// function declaration 2
test = () => {
}
}
The second way creates an instance function
class Node {
test = () => {
}
}
is the same as
class Node {
construtor() {
this.test = () => {
};
}
}
So effectively it is created a new function, bound to the new object's instance, every time an object is created. It's just a shorter way to say it.
The advantage is you can pass those methods to a callback. Example
class Node {
constructor(name) {
this.name = name;
}
test1 = () => {
console.log(this.name);
}
test2() {
console.log(this.name);
}
}
const node = new Node('foo');
setTimeout(node.test1); // works
setTimeout(node.test2); // fails will have wrong this
setTimeout(node.test2.bind(node)); // works
Note that when to use an arrow function on a method and when not to is really up to the situation
const node = new Node('foo');
someElement.addEventListener(click, node.test1);
In the case above when the event listener is called this will reference node. If that's what you want great but event listeners have this set to the element they're attached to, in this case someElement so you lose that. If you didn't need it great. If you did need it then an arrow function was the wrong thing to use.
I currently have the hidden attribute of one of my divs binded to a boolean in my typescript. But, when I am changing the value of the boolean in one of my function calls nested within an object the dom is not updating on the front end?
typescript
hideSymbols = true;
bindings = {
enter: {
key: 13,
handler: function() {
console.log('enter pressed');
this.hideSymbols = !this.hideSymbols;
console.log(this.hideSymbols);
}
}
};
html
<div [hidden]="hideSymbols">
<button id="equalsBtn" class="symbolBtn">=</button>
<button id="impliesBtn" class="symbolBtn">=></button>
</div>
It works if I am not making the call in this handler but I need to in order for my ngx-quill instance to update how the enter key works. Essentially, why is hideSymbols getting updated but on my web view the element does not disappear and reappear?
Make that:
handler: () => {...
...rather than use function. A function defined using function has its own this.
I'm not sure this is the whole problem without more context, but it's probably at least part of the problem.
this.hideSymbols = !this.hideSymbols; is executing in the wrong scope.
This is what you have:
var result1 = null;
var exhibitA = {
execute: function(){
this.result1 = "hello";
}
};
exhibitA.execute();
console.log({ exhibitA, result1 });
This is what you want:
var result2 = null;
var exhibitB = {
execute: () => {
this.result2 = "hello";
}
};
exhibitB .execute();
console.log({ exhibitB, result2 });
I've read in several places that the key difference is that this is lexically bound in arrow functions. That's all well and good, but I don't actually know what that means.
I know it means it's unique within the confines of the braces defining the function's body, but I couldn't actually tell you the output of the following code, because I have no idea what this is referring to, unless it's referring to the fat arrow function itself....which doesn't seem useful.
var testFunction = () => {
console.log(this)
};
testFunction();
Arrow functions capture the this value of the enclosing context
function Person(){
this.age = 0;
setInterval(() => {
this.age++; // |this| properly refers to the person object
}, 1000);
}
var p = new Person();
So, to directly answer your question, this inside your arrow function would have the same value as it did right before the arrow function was assigned.
In order to provide the big picture I'm going to explain both, dynamic and lexical binding.
Dynamic Name Binding
this refers to the object the method is called on. This is a regularly to be read sentence on SO. But it is still only a phrase, pretty abstract. Is there a corresponding code pattern to this sentence?
Yes there is:
const o = {
m() { console.log(this) }
}
// the important patterns: applying methods
o.m(); // logs o
o["m"](); // logs o
m is a method because it relies on this. o.m() or o["m"]() means m is applied to o. These patterns are the Javascript translation to our famous phrase.
There is another important code pattern that you should pay attention to:
"use strict";
const o = {
m() { console.log(this) }
}
// m is passed to f as a callback
function f(m) { m() }
// another important pattern: passing methods
f(o.m); // logs undefined
f(o["m"]); // logs undefined
It is very similar to the previous pattern, only the parenthesis are missing. But the consequences are considerable: When you pass m to the function f, you pull outm of its object/context o. It is uprooted now and this refers to nothing (strict mode assumed).
Lexical (or Static) Name Binding
Arrow functions don't have their own this/super/arguments binding. They inherit them from their parent lexical scope:
const toString = Object.prototype.toString;
const o = {
foo: () => console.log("window", toString.call(this)),
bar() {
const baz = () => console.log("o", toString.call(this));
baz();
}
}
o.foo() // logs window [object Window]
o.bar() // logs o [object Object]
Apart from the global scope (Window in browsers) only functions are able to form a scope in Javascript (and {} blocks in ES2015). When the o.foo arrow function is called there is no surrounding function from which baz could inherit its this. Consequently it captures the this binding of the global scope which is bound to the Window object.
When baz is invoked by o.bar, the arrow function is surrounded by o.bar (o.bar forms its parent lexical scope) and can inherit o.bar's this binding. o.bar was called on o and thus its this is bound to o.
Hope this code show could give you clearer idea. Basically, 'this' in arrow function is the current context version of 'this'. See the code:
// 'this' in normal function & arrow function
var this1 = {
number: 123,
logFunction: function () { console.log(this); },
logArrow: () => console.log(this)
};
this1.logFunction(); // Object { number: 123}
this1.logArrow(); // Window
Arrow function this is pointing to the surrounding parent in Es6, means it doesn't scope like anonymous functions in ES5...
It's very useful way to avoid assigning var self to this which is widely used in ES5...
Look at the example below, assigning a function inside an object:
var checkThis = {
normalFunction: function () { console.log(this); },
arrowFunction: () => console.log(this)
};
checkThis.normalFunction(); //Object {}
checkThis.arrowFunction(); //Window {external: Object, chrome: Object, document: document, tmpDebug: "", j: 0…}
You can try to understand it by following the way below
// whatever here it is, function or fat arrow or literally object declare
// in short, a pair of curly braces should be appeared here, eg:
function f() {
// the 'this' here is the 'this' in fat arrow function below, they are
// bind together right here
// if 'this' is meaningful here, eg. this === awesomeObject is true
console.log(this) // [object awesomeObject]
let a = (...param) => {
// 'this is meaningful here too.
console.log(this) // [object awesomeObject]
}
so 'this' in fat arrow function is not bound, means you can not make anything bind to 'this' here, .apply won't, .call won't, .bind won't. 'this' in fat arrow function is bound when you write down the code text in your text editor. 'this' in fat arrow function is literally meaningful here. What your code write here in text editor is what your app run there in repl. What 'this' bound in fat arror will never change unless you change it in text editor.
Sorry for my pool English...
Arrow function never binds with this keyword
var env = "globalOutside";
var checkThis = {env: "insideNewObject", arrowFunc: () => {
console.log("environment: ", this.env);
} }
checkThis.arrowFunc() // expected answer is environment: globalOutside
// Now General function
var env = "globalOutside";
var checkThis = {env: "insideNewObject", generalFunc: function() {
console.log("environment: ", this.env);
} }
checkThis.generalFunc() // expected answer is enviroment: insideNewObject
// Hence proving that arrow function never binds with 'this'
this will always refer to the global object when used inside an arrow function. Use the regular function declaration to refer to the local object. Also, you can use the object name as the context (object.method, not this.method) for it to refer to the local object instead of the global(window).
In another example, if you click the age button below
<script>
var person = {
firstName: 'John',
surname: 'Jones',
dob: new Date('1990-01-01'),
isMarried: false,
age: function() {
return new Date().getFullYear() - this.dob.getFullYear();
}
};
var person2 = {
firstName: 'John',
surname: 'Jones',
dob: new Date('1990-01-01'),
isMarried: false,
age: () => {
return new Date().getFullYear() - this.dob.getFullYear();
}
};
</script>
<input type=button onClick="alert(person2.age());" value="Age">
it will throw an exception like this
×JavaScript error: Uncaught TypeError: Cannot read property
'getFullYear' of undefined on line 18
But if you change person2's this line
return new Date().getFullYear() - this.dob.getFullYear();
to
return new Date().getFullYear() - person2.dob.getFullYear();
it will work because this scope has changed in person2
Differences between arrow functions to regular functions: (taken from w3schools)
With arrow functions there are no binding of this.
In regular functions the this keyword represented the object that called the function, which could be the window, the document, a button or whatever.
With arrow functions the this keyword always represents the object that defined the arrow function.
// Regular Function:
hello = function() {
document.getElementById("demo").innerHTML += this;
}
// The window object calls the function:
window.addEventListener("load", hello);
// A button object calls the function:
document.getElementById("btn").addEventListener("click", hello);
// -------------------------------------------
// Arrow function
hello2 = () => {
document.getElementById("demo2").innerHTML += this;
}
// The window object calls the function:
window.addEventListener("load", hello2);
// A button object calls the function:
document.getElementById("btn2").addEventListener("click", hello2);
<p><i>With a regular function this represents the <b>object that calls the function</b>:</i></p>
<button id='btn'>click me regular function</button>
<p id="demo">Regular function: </p>
<hr>
<p><i>With arrow function this represents the <b>owner of the function(=the window object)</b>:</i></p>
<button id='btn2'>click me arrow function</button>
<p id="demo2">Arrow function: </p>
A related issue:
Came from - Why can't I access `this` within an arrow function?
We know below from here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
Does not have its own bindings to this or super, and should not be used as methods.
Arrow functions establish "this" based on the scope the Arrow function is defined within.
Had a issue with this using arrow functions, so created a class (can be function), and class variable is accessed in arrow function, thereby achieved smaller functions using arrow functions without function keyword:
class MyClassOrFunction {
values = [];
size = () => this.values.length;
isEmpty = () => this.size() === 0;
}
let obj = new MyClassOrFunction();
obj.size(); // function call here
You can also have a getter like this, that does not have function keyword, but a bit longer due to return statement, also can access other member functions:
class MyClassOrFunction {
values = [];
size = () => this.values.length;
get length() { return this.size(); }
}
let obj = new MyClassOrFunction();
obj.length; // NOTE: no function call here
I have a function in node.js that looks like this:
exports.getAllFlights = function(getRequest) {
// this is the package from npm called "async"
async.map(clients, getFlight, function(err, results) {
getRequest(results);
});
}
The variable clients should be a JSON that looks like this:
{'"A4Q"': 'JZA8187', "'B7P"': 'DAL2098' }.
I expect that the map function will pass the individual indices of the array of the variable clients to getFlight. However, instead it passed the values of that each(ex: 'DAL2098', 'JZA8187' and so on).
Is this the expected functionality? Is there a function in async that will do what I want?
The signature of getFlight is getFlight(identifier, callback). Identifier is what is currently messed up. It returns callback(null, rtn). Null reprsents the nonexistence of an error, rtn represents the JSON that my function produces.
Yes, that's the expected result. The documentation is not very clear but all iterating functions of async.js pass the values of the iterable, not the keys. There is the eachOf series of functions that pass both key and value. For example:
async.eachOf(clients, function (value, key, callback) {
// process each client here
});
Unfortunately there is no mapOf.
If you don't mind not doing things in parallel you can use eachOfSeries:
var results = [];
async.eachOfSeries(clients, function (value, key, callback) {
// do what getFlight needs to do and append to results array
}, function(err) {
getRequest(results);
});
Another (IMHO better) workaround is to use proper arrays:
var clients = [{'A4Q': 'JZA8187'},{'B7P': 'DAL2098'}];
Then use your original logic. However, I'd prefer to use a structure like the following:
var clients = [
{key: 'A4Q', val: 'JZA8187'},
{key: 'B7P', val: 'DAL2098'}
];
First create a custom event. Attach a listener for return data. then process it.
var EventEmitter = require('events');
var myEmitter = new EventEmitter();
myEmitter.emit('clients_data',{'"A4Q"': 'JZA8187'}); //emit your event where ever
myEmitter.on('clients_data', (obj) => {
if (typeof obj !=='undefined') {
if (obj.contructor === Object && Object.keys(obj).lenth == 0) {
console.log('empty');
} else {
for(var key in obj) {
var value = obj[key];
//do what you want here
}
}
}
});
Well, you need to format your clients object properly before you can use it with async.map(). Lodash _.map() can help you:
var client_list = _.map(clients, function(value, key) {
var item = {};
item[key] = value;
return item;
});
After that, you will have an array like:
[ { A4Q: 'JZA8187' }, { B7P: 'DAL2098' } ]
Then, you can use async.map():
exports.getAllFlights = function(getRequest) {
async.map(client_list, getFlight, function(err, results) {
getRequest(results);
});
};