Is there a way to check if a variable has been declared but not assigned a value? I've looked at similar questions and they all seem to recommend using
typeof myVar !== 'undefined'
However, this always returns false because even when declared the variable is still undefined. Here are the results I am aiming for:
var myVar; // Variable is declared. Test should return TRUE
//var myVar; // Variable not declared. Test should return FALSE
Regrettably, using `typeof` will give you undefined in both scenarios. The only method I am aware of to determine if a variable is truly defined is by utilizing try-catch. Give this a shot:
var x;
var xExists=true;
var yExists=true;
try{x}catch(e){xExists=false}
try{y}catch(e){yExists=false}
console.log(xExists, yExists);
If you wish to determine if a global variable or property exists, one way is to use the hasOwnProperty method. For example, window.hasOwnProperty('myVar') or object.hasOwnProperty('myVar') will return true if the variable/property has been defined.
The typeof operator alone may not provide accurate information as it will only return 'undefined' for variables that have not been defined at all. To check if a variable is defined but not assigned a value, you can combine it with another technique. However, be aware that this approach may still result in false positives if a variable was initially assigned a value of undefined.
For local variables (non-global), you cannot reference their objects directly. Attempting to call an undefined variable should trigger an exception. You can use the following code snippet to test for this:
function isDefined(variableName) {
try {
eval(variableName);
} catch (ex) {
return false;
}
return true;
}
The use of eval in this function ensures that any exceptions occur within the try-catch block and not when isDefined is called. It is important to note that using this method in production environments is strongly discouraged.
In light of these considerations, it raises the question of whether checking for the existence of a variable is truly necessary in your context.
To check for the existence of a variable in the current context, you can utilize the in operator:
var myVarDeclared;
var myVarDefined = '';
var myVarDeclaredTest = 'myVarDeclared' in this;
var myVarDefinedTest = 'myVarDefined' in this;
var notDeclaredTest = 'notDeclared' in this;
document.write('myVarDeclared test: ' + myVarDeclaredTest + '<br>'); //true
document.write('myVarDefined test: ' + myVarDefinedTest + '<br>'); //true
document.write('notDeclared test: ' + notDeclaredTest + '<br>'); //falseI'm currently working on a project that involves utilizing the react framework. My intention is to incorporate the owl-carousel, however, I've encountered issues as it fails to function properly. The following error keeps popping up: OwlCarousel ...
I am currently incorporating openlayers into my ionic app and working on placing users on the map. I have encountered a problem as I am unsure how to apply custom CSS styling to the user element, which is not directly present in the HTML. In the screenshot ...
Currently, I have a FlatList that I am attempting to filter using multiple inputs from dropdown selectors. Here is the code snippet for my FlatList component: <FlatList style={styles.list} data={data.filter(filteredUsers)} ...
Why doesn't success respond? Here is the code I've used: Client-side code: function add(){ var values = formserial(addd); var tok = "abc", var url= 'http://localhost:8181/add'; $.ajax({ type: "POST", ...
I am currently executing Jasmine tests within a SystemJS and Typescript environment (essentially a plunk setup that is designed to be an Angular 2 testing platform). Jasmine is being deliberately utilized as a global library, rather than being imported vi ...
I'm currently in the process of developing an angularJS application. Below is a snippet of my PHP code: <label class="item item-input item-select"> <div class="input-label">Do you possess the right to work in the UK?</div> & ...
My webpage features 6 different images, including 4 of various balls such as basketball and baseball. One image is of a truck, while the last one is random. On the page, there is a form with two radio buttons allowing users to select which type of image c ...
I am facing difficulties with communication between my application and a module that I have developed. Below is the AngularJS module that I created. (function (document, window) { 'use strict'; var piCart = angular.module('piCart& ...
I currently have the following piece of code: <div>{{animalType}}</div> This outputs dog. Is there a way to conditionally add an 's' if the value of animalType is anything other than dog? I attempted the following, but it did not ...
I am currently working on a threejs application that requires updating a texture in each frame. The output encoding of the THREE.WebGLRenderer is set to sRGB. Upon setting the texture encoding to sRGB, I observed that the rendering result is accurate. How ...
I have an array let colorsArr = ["red", "green", "purple", "pink", "black"]; Is there a way to retrieve a specific number of elements from the END and move them to the BEGINNING of the array? Desired result: example 1: //move last 3 elements let expec ...
I just finished setting up a basic Express NodeJS server. My goal is to save the value of the setting returned from mongoose.find() as a variable in res.render, but every time I try, it throws an error saying Cannot read property of undefined. Here' ...
SCENARIO: I have downloaded the repository from this link: https://github.com/meanjs/mean After following and executing all instructions, I ran the command $ npm start and encountered the following error: ERROR: Could not connect to MongoDB! { Mon ...
I've been developing a hamburger menu for my website, and it seems to be working fine until I click on a link. Once I navigate to a new page, the menu stops responding. To fix this issue, I attempted to use window.location.reload, which worked in Chro ...
How can I adjust the value of a canvas arc? The input accepts larger values, but not smaller ones. var btn = document.querySelector(".btn"); btn.addEventListener("click", () => { var inputVal = document.querySelector(&q ...
I am experimenting with making a spinning wheel rotate as the user drags the mouse over it. The wheel consists of 3 pieces, so I have individually mapped each image to target the specific section of the wheel that I want to rotate. Currently, it is funct ...
I am facing an issue with my TreePanel where I want to preselect a specific node when loading it. The nodes are fetched from a remote json file and the tree structure loads correctly. However, the selected node is not getting highlighted and Firebug is sho ...
Currently experimenting with the latest version of q.js to integrate promises into my ajax calls without utilizing node.js at all. I retrieved the most recent version from https://github.com/kriskowal/q and only included q.js in my project. While testing, ...
I've been immersed in writing tests for the past few weeks. In my workplace, we utilize Mocha as our test runner and Chai for assertions, with Sinon for creating stubs. However, there's a recurring issue that's been bothering me. I've w ...
Is there a way to retrieve the data from an ajax request in PHP? I am able to successfully return a string of data using ajax, but I'm having trouble accessing the variable passed as a json object. How can I access a json object that only contains one ...