I used C++ before and I realized that pointers were very helpful. Is there anything in javascript that acts like a pointer? Does javascript have pointers? I like to use pointers when I want to use something like:
var a = 1;
var b = "a";
document.getElementById(/* value pointed by b */).innerHTML="Pointers";
I know that this is an extremely simple example and I could just use a, but there are several more complex examples where I would find pointers very useful. Any ideas?
No, JS doesn't have pointers.
Objects are passed around by passing a copy of a reference. The programmer cannot access any C-like "value" representing the address of an object.
Within a function, one may change the contents of a passed object via that reference, but you cannot modify the reference that the caller had because your reference is only a copy:
var foo = {'bar': 1};
function tryToMungeReference(obj) {
obj = {'bar': 2}; // won't change caller's object
}
function mungeContents(obj) {
obj.bar = 2; // changes _contents_ of caller's object
}
tryToMungeReference(foo);
foo.bar === 1; // true - foo still references original object
mungeContents(foo);
foo.bar === 2; // true - object referenced by foo has been modified
var a = {}; var b = { c:a }; it is safe to say that b.c does not hold a copy of a, but a value representing the location of a in memory. In this case there would be 3 spaces allocated in memory: one for a of type Object, one for b of type Object, and one for b.c of type Int holding a value of the location in memory of a. –
Pinder You bet there are pointers in JavaScript; objects are pointers.
//this will make object1 point to the memory location that object2 is pointing at
object1 = object2;
//this will make object2 point to the memory location that object1 is pointing at
function myfunc(object2){}
myfunc(object1);
If a memory location is no longer pointed at, the data there will be lost.
Unlike in C, you can't see the actual address of the pointer nor the actual value of the pointer, you can only dereference it (get the value at the address it points to.)
I just did a bizarre thing that works out, too.
Instead of passing a pointer, pass a function that fills its argument into the target variable.
var myTarget;
class dial{
constructor(target){
this.target = target;
this.target(99);
}
}
var myDial = new dial((v)=>{myTarget = v;});
This may look a little wicked, but works just fine. In this example I created a generic dial, which can be assigned any target in form of this little function "(v)=>{target = v}". No idea how well it would do in terms of performance, but it acts beautifully.
due to the nature of JS that passes objects by value (if referenced object is changed completely) or by reference (if field of the referenced object is changed) it is not possible to completely replace a referenced object.
However, let's use what is available: replacing single fields of referenced objects. By doing that, the following function allows to achieve what you are asking for:
function replaceReferencedObj(refObj, newObj) {
let keysR = Object.keys(refObj);
let keysN = Object.keys(newObj);
for (let i = 0; i < keysR.length; i++) {
delete refObj[keysR[i]];
}
for (let i = 0; i < keysN.length; i++) {
refObj[keysN[i]] = newObj[keysN[i]];
}
}
For the example given by user3015682 you would use this function as following:
replaceReferencedObj(foo, {'bar': 2})
Assigning by reference and arrays.
let pizza = [4,4,4];
let kebab = pizza; // both variables are references to shared value
kebab.push(4);
console.log(kebab); //[4,4,4,4]
console.log(pizza); //[4,4,4,4]
Since original value isn't modified no new reference is created.
kebab = [6,6,6,6]; // value is reassigned
console.log(kebab); //[6,6,6,6]
console.log(pizza); //[4,4,4,4]
When the compound value in a variable is reassigned, a new reference is created.
Yes, in fact, all objects and arrays are pointers. If I have to take your example literally, then, you only need to turn the variables to be of object types E.g.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Pointers</title>
</head>
<body>
<h1>Asynchronous JavaScript</h1>
<!--the pointer with class b--->
<p>Our City</p>
<p id="variableA" >Minnesota</p>
<script src="pointers.js" ></script>
</body>
</html>
Then the pointers.js:
var a = {name: "todos", data: {pointerOne: 'Minnesota', pointerTwo: 'NewYork'}};
var b = a;//pointer to a
setTimeout(()=>{
//using timeout to see our pointer at work!
b.data.pointerOne = 'Nairobi';
document.getElementById('variableA').innerHTML = a.data.pointerOne;
}, 3000);
The major difference in JavaScript, is the way the pointers are actually presented or the lack of a direct way to identify pointers. In C, Golang, and C# pointers are made using the ‘&’ operator.
This is the basic. After getting this concept, you can now develop it further into function pointers, linked lists, or use it like structs; and do JavaScript the C++ way. Though I would advice you to use the current JavaScript convention in naming your variables. i.e
const a = {name: "todos", data: {pointerOne: 'Minnesota', pointerTwo: 'NewYork'}};
const b = a;//pointer to a
Technically JS doesn't have pointers, but I discovered a way to imitate their behavior ;)
var car = {
make: 'Tesla',
nav: {
lat: undefined,
lng: undefined
}
};
var coords: {
center: {
get lat() { return car.nav.lat; }, // pointer LOL
get lng() { return car.nav.lng; } // pointer LOL
}
};
car.nav.lat = 555;
car.nav.lng = 777;
console.log('*** coords: ', coords.center.lat); // 555
console.log('*** coords: ', coords.center.lng); // 777
car. It has nothing to do with pointers or imitating and is known as the scope –
Brittaney © 2022 - 2024 — McMap. All rights reserved.
swapfunction like in C :) – Kilometer[a, b] = [b, a];– Antoineantoinettaaandbare references :) I mean you can't swap the values of primitives (for example numbers) in another function but you can do it easily in C with pointers – Kilometerswap(a, b), you[a, b] = [b, a]. (And no, the behaviour doesn’t depend on whataandbcontain.) – Antoineantoinetta[a, b] = [b, a]everywhere. I mean that I can't write this in a some functionswap. For example in C it costs nothing to add additional log about swapping the values of two variables but in JS I have to write this additional log everywhere when I swap the values of two variables – Kilometer