Are there any drawbacks on using this kind of JavaScript code? [duplicate] - javascript

This question already has answers here:
Why can I change a constant object in javascript
(12 answers)
Closed 2 years ago.
Consider the following JavaScript code:
const myObject = {
innerValue: 'test'
}
myObject.innerValue = 'I can still change this, this is not a constant';
If I run this code, the browser outputs no errors, I presume, because only the outer object myObject is a constant and its properties are not. But how valid is this JavaScript code? I mean, are there any negative drawbacks to writing something like this?

Looks like you already understand that the variable myObject is const, but the object to which it refers is not. That's very much by design for JavaScript. Using const the way you did does not protect you from modifying the object.
You could use a property to protect the value of innerValue.
const myObject = {};
Object.defineProperty(myObject, 'innerValue', {
value: 'test',
writable: false
});
console.log(myObject.innerValue);
myObject.innerValue = 'this is not an error but will not change the value';
console.log(myObject.innerValue);

Related

JavaScript function not working when passing variable instead of hard coded value [duplicate]

This question already has answers here:
Using a string to access a variable
(3 answers)
Closed 3 years ago.
I have a JS function as below
// A simple array where we keep track of things that are filed.
filed = [];
function fileIt(thing) {
// Dynamically call the file method of whatever 'thing' was passed in.
thing.file();
// Mark as filed
filed.push(thing);
}
Now, function fileIt(thing) is working well when called as below
fileIt(AuditForm);
Whereas, its giving error at line thing.file(); when i am trying to pass a variable like below
var formID = obj.id;
fileIt(formID);
Variable formID has same value and i.e. "AuditForm" what's wrong here. Kindly suggest.
If obj.id is the string AuditForm, then you have no choice but to use dynamic property notation on the global window object, or use eval if you didn't declare AuditForm with var on the global scope:
If you declare AuditForm with var on the global scope:
fileIt(window[formID]);
If you don't:
fileIt(eval(formID));
Do note that eval is a very poor option, as if obj.id can be interpreted as other code, e.g. another eval call which will be evaluated, then malicious operations can be performed. Example:
const obj = {
id: "eval('alert(\"Inside an eval script!\")')"
};
eval(obj.id);

Javascript: set object variable to inner object instance [duplicate]

This question already has answers here:
Self-references in object literals / initializers
(30 answers)
Closed 6 years ago.
I'm trying to create a class to better organize my code, I decided to go the object literal route
var MyClass = {
}
I'm currently running into issues migrating some of my functionality though, previously I had global scope variables Channels for example set to an object instance. Is there a way to still do this in javascript within a class without moving the object into global scope?
var Prime = {
Channel: new __Prime__Channel(),
//The object in question
__Prime__Channel: function() {
this.Property = Value;
},
}
this throws
Undefined reference __Prime__Channel() at line 2
In global scope you could do
var Channel = new __Prime__Channel();
function __Prime__Channel() {
this.Property = Value;
}
without any errors
An object is no class, please ensure you're using the right terminology.
With an object literal it is not possible to do what you want, the only way is
var Prime = {
__Prime__Channel: function() {
this.Property = Value;
}
}
Prime.Channel = new Prime.__Prime__Channel();
However, maybe an object literal is the wrong pattern anyway. If you want to avoid global variables, look into the module pattern and IIFEs.

What is this js syntax called? [duplicate]

This question already has answers here:
Javascript object bracket notation ({ Navigation } =) on left side of assign
(5 answers)
Closed 6 years ago.
I'm not sure if there is a specific name for this way to pull object properties into their own variable? If there is a name, does anyone know?
var object = {
something: 'a string',
another: 'another string',
}
var { something, another } = object;
This is called object destructuring.
Object destructuring, read up on it here:
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment
A lot of this new syntax isn't fully implemented in all js engines, so be careful when using it!
If you want to learn more about it, checkout this tutorial:
https://www.eventbrite.com/engineering/learning-es6-destructuring/

Call sub function in object with variable brackets [duplicate]

This question already has answers here:
Accessing nested JavaScript objects and arrays by string path
(44 answers)
Closed 7 years ago.
var myObject = {
sub: {
myFunction: function() {
console.log('check');
}
}
}
var callFn = 'sub.myFunction'; // I want it to solve it here, whats going wrong?
myObject[callFn](); // Works, but not with 'sub.myFunction'
myObject.sub.myFunction(); // This works ofc.
I need a generic solution. Can someone explain why the sub.myFunction does not work? Does anyone have a workaround to solve this?
The . character is a valid part of a property name in javascript as long as you enclose it in quotes.
What that means to you:
myObject["sub.myFunction"] - looks for a property named sub.myFunction on myObject. It does not look for a property named myFunction on the sub property of myObject.
myObject.sub.myFunction - does not have the . in quotes so it's treated as a property accessor. That means we look for a myFunction property on the sub property of myObject.

Javascript Object context change and eval [duplicate]

This question already has answers here:
calling eval() in particular context
(18 answers)
Closed 7 years ago.
I have some trouble using the Object properties. I want to evaluate an expression within the myObject context to avoid using the myObject.property.
My final goal is to evaluate a more complex expression like 'property1+property2' instead of 'myObject.property1+myObject.property2'.
I have tried the call method to change the context but, it doesn't seem to see the Object properties in the passed context(i.e. the Object containing the properties)(see the last line of the code below generating the error).
var myObject = {
property1: 20,
property2: 5000
};
print(myObject.property1); // return 20
print(eval.call(myObject,property1)); // ReferenceError: property1 is not defined
Is there any way to use the Object properties without the usage of this. or myObject. prefix?
Well, there's with statement which is deprecated and you probably shouldn't use it too much, but in this case maybe it wouldn't be considered harmful:
with(myObject){
console.log( property1 ); // 20
console.log( eval('property1') ); //20
console.log( eval('property1+property2') ); // 5020
}
http://jsfiddle.net/f7d1b79b/1/

Categories

Resources