So...there is this type in js called Map and it is really nice...it is faster than an Object for iterations and calculations so I really like it. However...you can't pass Maps around as you could with objects.
I know that I could turn Map into JSON but it is costly to do so and it kind of looses the point of using Maps in the first place.
JSON format is not meant for Maps...it was meant for Objects.
So...lets move from the JSON format a little.
Is there a way for me to serialize a Map into a string in any way so that I can then do the opposite - from said serialized Map to end up with a Map
Preferably this method should be as easy to perform as JSON.stringify or its counterpart JSON.parse.
I want to use Map as it is faster and better but I need to send my data as string. The format of the string is not important as long as I can parse it back into a Map
-- edit: Added the missing JSON Stringify function during serialization -
There is a native way of doing this in Javascript.
the Map object has a method called entries() that returns an iterator that will yield each entry in the map as an array with 2 values in it. It will look like [key, value].
To avoid writing any looping code yourself, you can use Array.from() which can consume an iterator and extract each item from it. Having that, the following will give you your Map object in a serialized form.
let mySerialMap = JSON.stringify(Array.from(myMap.entries()))
console.log(mySerialMap)
Now, that's not even the coolest part. But before getting there, let me show you one possible way of using the Map constructor.
let original = new Map([
['key1', 'the-one-value']
['second-key, 'value-two']
])
The array of array that you can see being passed to the Map object is in the same format as what you get from using Array.from(myMap.entries()).
So you can rebuild you map in a single line using the following sample code:
let myMap = new Map(JSON.parse(mySerialMap))
And you can use myMap as you would any Map.
let myMap = new Map(JSON.parse(mySerialMap))
let first = myMap.get('some-key')
myMap.set('another-key', 'some value')
I guess the whole point of Maps/Dictionaries is that you can use objects as keys in them, so:
let a = {}, b = {}, m = new Map();
m.set(a,b);
m.get(a); // b
So you get b since you have a reference on a. Let's say you serialize the Map by creating an Array of arrays, and stringify that to json:
function serialize (map) {
return JSON.stringify([...map.entries()])
}
let s = serialize(m); // '[[{}, {}]]'
// '[[<key>, <val>], … ]'
Than you could:
let m2 = JSON.parse(s).reduce((m, [key, val])=> m.set(key, val) , new Map());
But the question now is: How to get a certain key? Since there does not exist any reference, due to the fact that all objects have been stringified and recreated, it is not possible to query a dedicated key.
So all that would only work for String keys, but that really takes most of power of maps, or in other words, reduces them to simple objects, what is the reason maps were implemented.
To #philipp's point, people who care about the serialization of the Map will probably prefer objects (leverages intuition, reduces '[]' arithmetic). Object.entries() and Object.fromEntries() can make that a bit more literate:
writeMe = new Map()
writeMe.set('a', [1])
writeMe.set('b', {myObjValue: 2})
// ▶ Map(2) {'a' => Array(1), 'b' => {…}}
written = JSON.stringify(Object.fromEntries(writeMe.entries()))
// '{"a":[1],"b":{"myObjValue":2}}'
read = new Map(Object.entries(JSON.parse(written)))
// ▶ Map(2) {'a' => Array(1), 'b' => {…}}
read.get("b")
// ▶ {myObjValue: 2}
I have a string like "car;bus;airplane;bike"
what is the fastest way to convert it into a set in JavaScript JSX?
Is there something like:
"car;bus;airplane;bike".split(';').toSet()?
So the set will have 'car', 'bus', 'airplane', 'bike' as elements
you can use
let x = new Set("car;bus;airplane;bike".split(';'));
Javscript Set can be initialised as
new Set([iterable]);
you can initialise it with an iterable object
Use:
new Set("car;bus;airplane;bike".split(';'))
Since you can construct a Set from an itrerable, you could go for:
const theString = "car;bus;airplane;bike";
const theSet = new Set(theString.split(";"));
You can use the Set constructor:
s = new Set("car;bus;airplane;bike".split(';'))
console.log(s.size);
console.log(s);
The constructor can take any iterable and convert the objects to be the set's elements.
Note that stackoverflow's snippet will not show the log of s but if you use chrome you can open the console and find the relevant result there.
You said "fastest" but all of the answers to date have used strings that they split into arrays and passed into the Set constructor. Cut out the middleman if you really meant "fastest":
const s = new Set(["car","bus","airplane","bike"]);
console.log(s.size);
console.log(s.has("bus"));
Not that the speed of this operation is likely to be significant.
const x = new Set('car;bus;bike'.split(';'));
console.log(x);
I have an ko.observable with an object that contains 3 arrays like this:
self.filter({ file: [], site: [], statut: [] })`
When I try to empty them it doesn't work. I tried
array = []
to empty them. Is it a problem with the observable?
You don't need all of your observable object's arrays to be observable to be able to update the UI, although I'd certainly (like the other answerer) advice you to do so.
I would like to explain however why it doesn't work.
Say you have the following code:
var originalObject = {
myArray: [1, 2, 3]
};
var myObservable = ko.observable(originalObject);
// Resetting the array behind knockout's back:
originalObject.myArray = [1, 2, 3, 4];
The last line changes a property of the object that was used to set the observable. There's no way for knockout to know you've updated your object. If you want knockout to reconsider the observable's vale, you have to tell it something's changed:
myObservable.valueHasMutated();
Now, normally, you update an observable by passing a new or updated variable to it like so:
myObservable(newValue);
Strangely, setting the observable with the same object again also works:
myObservable(originalObject);
This is why:
Internally, knockout compares the newValue to the value it currently holds. If the values are the same, it doesn't do anything. If they're different, it sets the new value and performs the necessary UI updates.
Now, if you're working with just a boolean or number, you'll notice knockout has no problems figuring out if the new value is actually different:
var simpleObservable = ko.observable(true);
simpleObservable.subscribe(function(newValue) {
console.log("Observable changed to: " + newValue);
});
simpleObservable(true); // Doesn't log
simpleObservable(false); // Does log
<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
For objects however, it behaves differently:
var myObject = { a: 1 };
var simpleObservable = ko.observable(myObject);
simpleObservable.subscribe(function(newValue) {
console.log("Observable changed to: " + JSON.stringify(newValue, null, 2));
});
simpleObservable(myObject); // Does log, although nothing changed
simpleObservable({b: 2 }); // Does log
<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
The subscription is triggered, even though we've used the exact same object to reset our observable! If you dig through knockout's source code, you'll see why. It uses this method to check if the new value is different:
var primitiveTypes = { 'undefined':1, 'boolean':1, 'number':1, 'string':1 };
function valuesArePrimitiveAndEqual(a, b) {
var oldValueIsPrimitive = (a === null) || (typeof(a) in primitiveTypes);
return oldValueIsPrimitive ? (a === b) : false;
}
Simply put: if the old value isn't a primitive value, it will just assume things have changed. This means we can update our originalObject, as long as we reset the observable.
originalObject.myArray.length = 0;
myObservable(originalObject);
Or, just as easy:
myObservable(Object.assign(originalObject, { myArray: [] });
A bit of a long answer, but I believe it's nice to know why stuff doesn't work, instead of only circumventing it. Even if simply using observableArrays and letting knockout optimize its work is the better solution!
You mention "emptying" an array. Note that that's different from "assigning a new, empty array to a variable". At any rate, if you want to "empty" an array:
For observableArray check the relevant docs, because they have a removeAll() utility method.
For emptying a plain javascript array, check this duplicate question that has various solutions, one of which is simply array.length = 0.
As a final note, if you're inside the view model, you might need to do self.filter() first to get the object inside the observable. So, for example:
self.filter().file.length = 0; // plain array method
However, since file, site, and statut are plain arrays (and not observableArrays) there will be no automatic updates in your UI. If they were observable arrays, you'd do:
self.filter().file.removeAll(); // assuming `file` has been made observable
What's the real difference between declaring an array like this:
var myArray = new Array();
and
var myArray = [];
There is a difference, but there is no difference in that example.
Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:
x = new Array(5);
alert(x.length); // 5
To illustrate the different ways to create an array:
var a = [], // these are the same
b = new Array(), // a and b are arrays with length 0
c = ['foo', 'bar'], // these are the same
d = new Array('foo', 'bar'), // c and d are arrays with 2 strings
// these are different:
e = [3] // e.length == 1, e[0] == 3
f = new Array(3), // f.length == 3, f[0] == undefined
;
Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.
As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.
The difference between creating an array with the implicit array and the array constructor is subtle but important.
When you create an array using
var a = [];
You're telling the interpreter to create a new runtime array. No extra processing necessary at all. Done.
If you use:
var a = new Array();
You're telling the interpreter, I want to call the constructor "Array" and generate an object. It then looks up through your execution context to find the constructor to call, and calls it, creating your array.
You may think "Well, this doesn't matter at all. They're the same!". Unfortunately you can't guarantee that.
Take the following example:
function Array() {
this.is = 'SPARTA';
}
var a = new Array();
var b = [];
alert(a.is); // => 'SPARTA'
alert(b.is); // => undefined
a.push('Woa'); // => TypeError: a.push is not a function
b.push('Woa'); // => 1 (OK)
In the above example, the first call will alert 'SPARTA' as you'd expect. The second will not. You will end up seeing undefined. You'll also note that b contains all of the native Array object functions such as push, where the other does not.
While you may expect this to happen, it just illustrates the fact that [] is not the same as new Array().
It's probably best to just use [] if you know you just want an array. I also do not suggest going around and redefining Array...
There is an important difference that no answer has mentioned yet.
From this:
new Array(2).length // 2
new Array(2)[0] === undefined // true
new Array(2)[1] === undefined // true
You might think the new Array(2) is equivalent to [undefined, undefined], but it's NOT!
Let's try with map():
[undefined, undefined].map(e => 1) // [1, 1]
new Array(2).map(e => 1) // "(2) [undefined × 2]" in Chrome
See? The semantics are totally different! So why is that?
According to ES6 Spec 22.1.1.2, the job of Array(len) is just creating a new array whose property length is set to the argument len and that's it, meaning there isn't any real element inside this newly created array.
Function map(), according to spec 22.1.3.15 would firstly check HasProperty then call the callback, but it turns out that:
new Array(2).hasOwnProperty(0) // false
[undefined, undefined].hasOwnProperty(0) // true
And that's why you can not expect any iterating functions working as usual on arrays created from new Array(len).
BTW, Safari and Firefox have a much better "printing" to this situation:
// Safari
new Array(2) // [](2)
new Array(2).map(e => 1) // [](2)
[undefined, undefined] // [undefined, undefined] (2)
// Firefox
new Array(2) // Array [ <2 empty slots> ]
new Array(2).map(e => 1) // Array [ <2 empty slots> ]
[undefined, undefined] // Array [ undefined, undefined ]
I have already submitted an issue to Chromium and ask them to fix this confusing printing:
https://bugs.chromium.org/p/chromium/issues/detail?id=732021
UPDATE: It's already fixed. Chrome now printed as:
new Array(2) // (2) [empty × 2]
Oddly enough, new Array(size) is almost 2x faster than [] in Chrome, and about the same in FF and IE (measured by creating and filling an array). It only matters if you know the approximate size of the array. If you add more items than the length you've given, the performance boost is lost.
More accurately: Array( is a fast constant time operation that allocates no memory, wheras [] is a linear time operation that sets type and value.
For more information, the following page describes why you never need to use new Array()
You never need to use new Object() in
JavaScript. Use the object literal {}
instead. Similarly, don’t use new Array(),
use the array literal []
instead. Arrays in JavaScript work
nothing like the arrays in Java, and
use of the Java-like syntax will
confuse you.
Do not use new Number, new String, or
new Boolean. These forms produce
unnecessary object wrappers. Just use
simple literals instead.
Also check out the comments - the new Array(length) form does not serve any useful purpose (at least in today's implementations of JavaScript).
In order to better understand [] and new Array():
> []
[]
> new Array()
[]
> [] == []
false
> [] === []
false
> new Array() == new Array()
false
> new Array() === new Array()
false
> typeof ([])
"object"
> typeof (new Array())
"object"
> [] === new Array()
false
> [] == new Array()
false
The above result is from Google Chrome console on Windows 7.
The first one is the default object constructor call. You can use it's parameters if you want.
var array = new Array(5); //initialize with default length 5
The second one gives you the ability to create not empty array:
var array = [1, 2, 3]; // this array will contain numbers 1, 2, 3.
I can explain in a more specific way starting with this example that's based on Fredrik's good one.
var test1 = [];
test1.push("value");
test1.push("value2");
var test2 = new Array();
test2.push("value");
test2.push("value2");
alert(test1);
alert(test2);
alert(test1 == test2);
alert(test1.value == test2.value);
I just added another value to the arrays, and made four alerts:
The first and second are to give us the value stored in each array, to be sure about the values. They will return the same!
Now try the third one, it returns false, that's because
JS treats test1 as a VARIABLE with a data type of array, and it treats test2 as an OBJECT with the functionality of an array, and
there are few slight differences here.
The first difference is when we call test1 it calls a variable without thinking, it just returns the values that are stored in this variable disregarding its data type!
But, when we call test2 it calls the Array() function and then it stores our "Pushed" values in its "Value" property, and the same happens when we alert test2, it returns the "Value" property of the array object.
So when we check if test1 equals test2 of course they will never return true, one is a function and the other is a variable (with a type of array), even if they have the same value!
To be sure about that, try the 4th alert, with the .value added to it; it will return true. In this case we tell JS "Disregarding the type of the container, whether was it function or variable, please compare the values that are stored in each container and tell us what you've seen!" that's exactly what happens.
I hope I said the idea behind that clearly, and sorry for my bad English.
There is no difference when you initialise array without any length. So var a = [] & var b = new Array() is same.
But if you initialise array with length like var b = new Array(1);, it will set array object's length to 1. So its equivalent to var b = []; b.length=1;.
This will be problematic whenever you do array_object.push, it add item after last element & increase length.
var b = new Array(1);
b.push("hello world");
console.log(b.length); // print 2
vs
var v = [];
a.push("hello world");
console.log(b.length); // print 1
There's more to this than meets the eye. Most other answers are correct BUT ALSO..
new Array(n)
Allows engine to reallocates space for n elements
Optimized for array creation
Created array is marked sparse which has the least performant array operations, that's because each index access has to check bounds, see if value exists and walk the prototype chain
If array is marked as sparse, there's no way back (at least in V8), it'll always be slower during its lifetime, even if you fill it up with content (packed array) 1ms or 2 hours later, doesn't matter
[1, 2, 3] || []
Created array is marked packed (unless you use delete or [1,,3] syntax)
Optimized for array operations (for .., forEach, map, etc)
Engine needs to reallocate space as the array grows
This probably isn't the case for older browser versions/browsers.
The first one is the default object constructor call.mostly used for dynamic values.
var array = new Array(length); //initialize with default length
the second array is used when creating static values
var array = [red, green, blue, yellow, white]; // this array will contain values.
The difference of using
var arr = new Array(size);
Or
arr = [];
arr.length = size;
As been discussed enough in this question.
I would like to add the speed issue - the current fastest way, on google chrome is the second one.
But pay attention, these things tend to change a lot with updates. Also the run time will differ between different browsers.
For example - the 2nd option that i mentioned, runs at 2 million [ops/second] on chrome, but if you'd try it on mozilla dev. you'd get a surprisingly higher rate of 23 million.
Anyway, I'd suggest you check it out, every once in a while, on different browsers (and machines), using site as such
As I know the diference u can find the slice(or the other funcitons of Array) like code1.and code2 show u Array and his instances:
code1:
[].slice; // find slice here
var arr = new Array();
arr.slice // find slice here
Array.prototype.slice // find slice here
code2:
[].__proto__ == Array.prototype; // true
var arr = new Array();
arr.__proto__ == Array.prototype; // true
conclusion:
as u can see [] and new Array() create a new instance of Array.And they all get the prototype functions from Array.prototype
They are just different instance of Array.so this explain why
[] != []
:)
There is no big difference, they basically do the same thing but doing them in different ways, but read on, look at this statement at W3C:
var cars = ["Saab", "Volvo","BMW"];
and
var cars = new Array("Saab", "Volvo", "BMW");
The two examples above do exactly the same. There is no need to use
new Array(). For simplicity, readability and execution speed, use the
first one (the array literal method).
But at the same time, creating new array using new Array syntax considered as a bad practice:
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new Array().
Use [] instead.
These two different statements both create a new empty array named
points:
var points = new Array(); // Bad
var points = []; // Good
These two different statements both create a new array containing 6
numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
The new keyword only complicates the code. It can also produce some
unexpected results:
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
So basically not considered as the best practice, also there is one minor difference there, you can pass length to new Array(length) like this, which also not a recommended way.
I've incurred in a weird behaviour using [].
We have Model "classes" with fields initialised to some value. E.g.:
require([
"dojo/_base/declare",
"dijit/_WidgetBase",
], function(declare, parser, ready, _WidgetBase){
declare("MyWidget", [_WidgetBase], {
field1: [],
field2: "",
function1: function(),
function2: function()
});
});
I found that when the fields are initialised with [] then it would be shared by all Model objects. Making changes to one affects all others.
This doesn't happen initialising them with new Array(). Same for the initialisation of Objects ({} vs new Object())
TBH I am not sure if its a problem with the framework we were using (Dojo)
Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.
Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.
Now let's see what are the difference
The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length.
For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.
The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.
Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).
I hope that has explained enough about the beast new Array() :)
I found a difference while using promises. While using array of promises (say arr, initialised as arr=[]), got an error in Promise.all(arr). Whereas when declared as arr = Array(), did not get compilation issues. Hope this helps.
I've found one difference between the two constructions that bit me pretty hard.
Let's say I have:
function MyClass(){
this.property1=[];
this.property2=new Array();
};
var MyObject1=new MyClass();
var MyObject2=new MyClass();
In real life, if I do this:
MyObject1.property1.push('a');
MyObject1.property2.push('b');
MyObject2.property1.push('c');
MyObject2.property2.push('d');
What I end up with is this:
MyObject1.property1=['a','c']
MyObject1.property2=['b']
MyObject2.property1=['a','c']
MyObject2.property2=['d']
I don't know what the language specification says is supposed to happen, but if I want my two objects to have unique property arrays in my objects, I have to use new Array().
Using the Array constructor makes a new array of the desired length and populates each of the indices with undefined, the assigned an array to a variable one creates the indices that you give it info for.