An “undefined variable” is different from the value undefined
.
An undefined variable:
var a; alert(b); // ReferenceError: b is not defined
A variable with the value undefined
:
var a; alert(a); // Alerts “undefined”
When a function takes an argument, that argument is always declared even if its value is undefined
, and so there won’t be any error. You are right about != null
followed by !== undefined
being useless, though.
Related Posts:
- JavaScript null check
- How to resolve TypeError: Cannot convert undefined or null to object
- How can I determine if a variable is ‘undefined’ or ‘null’?
- What is the difference between null and undefined in JavaScript?
- Is there a standard function to check for null, undefined, or blank variables in JavaScript?
- How to check for an undefined or null variable in JavaScript?
- How can I check for “undefined” in JavaScript?
- How can I check for “undefined” in JavaScript? [duplicate]
- How do I check for null values in JavaScript?
- How can I check for an empty/undefined/null string in JavaScript?
- Check if object exists in JavaScript
- Origin null is not allowed by Access-Control-Allow-Origin
- How to check if a variable is not null?
- Cannot read property ‘length’ of null (javascript)
- JavaScript check if variable exists (is defined/initialized)
- How to check ‘undefined’ value in jQuery
- How can I unset a JavaScript variable?
- How to add a class to a given element?
- How to make a word underline in Markdown
- Creating multiline strings in JavaScript
- What is lexical scope?
- Remove element by id
- await is only valid in async function
- Remove duplicate values from JS array [duplicate]
- How to read an external local JSON file in JavaScript?
- SyntaxError: Unexpected token o in JSON at position 1
- Remove duplicate values from JS array
- How can I convert a string to boolean in JavaScript?
- React Hook Warnings for async function in useEffect: useEffect function must return a cleanup function or nothing
- Change Background color (css property) using Jquery
- jQuery get textarea text
- Short circuit Array.forEach like calling break
- display variable image using cookies
- Find an element in DOM based on an attribute value
- Use of $parse in angular js
- With jQuery, how do I capitalize the first letter of a text field while the user is still editing that field?
- Pass a JavaScript function as parameter
- Cannot read property ‘getContext’ of null, using canvas
- Difference between “module.exports” and “exports” in the CommonJs Module System
- Local package.json exists, but node_modules missing
- Dealing with float precision in Javascript
- JSON.parse() not working
- GulpUglifyError:Unable to minify JavaScript
- Uncaught TypeError: Cannot read property ‘toUpperCase’ of undefined
- why angularjs ng-repeat not working
- jQuery remove options from select
- Convert UTC Epoch to local date
- How to execute a JavaScript function when I have its name as a string
- How to get the ASCII value in JavaScript for the characters [duplicate]
- HTML img onclick Javascript
- Clear the cache in JavaScript
- error Unable to get property ‘display’ of undefined or null reference
- Is there a php echo/print equivalent in javascript
- The entity name must immediately follow the ‘&’ in the entity reference
- How can I use goto in Javascript?
- Javascript Function that returns true if a letter?
- How do I know which version of Javascript I’m using?
- getContext is not a function
- lodash multi-column sortBy
- navigator.geolocation.getCurrentPosition sometimes works sometimes doesn’t
- Expected ‘this’ to be used by class method
- Strip HTML from Text JavaScript
- Calling a JavaScript function in another js file
- JavaScript code not running in HTML5 document
- What is the cause for “angular is not defined”
- Unexpected token < in first line of HTML
- ngular is automatically adding ‘ng-invalid’ class on ‘required’ fields
- How to set “style=display:none;” using jQuery’s attr method?
- Await is a reserved word error inside async function
- Javascript WP Object Documentation?
- Show only images and videos in a wp.media window
- WordPress Rest API
- Instantiate TinyMCE via JS ( rather than PHP/wp_editor )
- Force core to use .dev JavaScript files
- TinyMCE Button to Insert Multiple Lines of Text?
- How to add extra attributes to the script tag added via wp_localize_script()
- Prevent escaping javascript in visual editor
- @wordpress/components Button variants not styled
- Only displaying zip files with wp media
- Adding a Javascript slideshow to the home page
- List of JS Events for Widgets?
- Authentication with the Rest API when using an External Application
- Open Featured Image Modal in WordPress Gutenberg onClick of a button
- Screen Resolution Detect
- Override do_item() Function to Add Extra Attribute to Scripts
- Is it possible to pass js variable to template file
- Automatically Add Defer or ASYNC to all JS files (no matter where they are located)
- Uncaught ReferenceError: jQuery is not defined – although jQuery is loaded [closed]
- How can I import one custom block into another?
- Gutenberg Blocks: how to determine an index of the current inner block?
- find out reason of “Updating failed” in Post-editor
- Getting a variable inside foreach from PHP to JS after localization
- How to import a JavaScript Library into WordPress
- How to change the link address from internal to external
- Uncaught TypeError: Cannot read property ‘add’ of undefined bp-plupload.min.js
- How to verify that a field has a value before adding a term to a taxonomy
- List unused javascript for easy removal
- Include Facebook Javascript SDK In WordPress
- How to load JavaScript modules with script_loader_tag hook?
- How to securely set dynamic HTML content with JavaScript?