Answers by Sentry
Answers and explanations to help developers understand concepts and avoid future problems. Learn why problems happen, not just how to resolve one instance of them.
Ask a questionJoin the discussion- Asynchronous Callbacks in JavaScript
- How do I check if an array includes a value in JavaScript?
- How do I Check for an Empty/Undefined/Null String in JavaScript?
- How do I check whether a checkbox is checked in jQuery?
- How do I copy to the clipboard in JavaScript?
- How do I Empty an Array in JavaScript?
- How do I get all of the unique values in a JavaScript array (remove duplicates)?
- How do I Format a Date in JavaScript?
- How do I get the Current Date in JavaScript?
- Get value of text input field using JavaScript
- How do I Make the First Letter of a String Uppercase in JavaScript?
- How do I refresh a page using JavaScript?
- How to round to at most two decimal places in JavaScript
- Capturing JavaScript Errors
- How do I check if an element is hidden in jQuery?
- Check if a variable is a string in JavaScript
- Fixing ChunkLoadErrors in Javascript
- Comments in JSON
- Warning: componentWillMount has been Renamed, and is not Recommended for Use
- Convert Unix timestamp to date and time in JavaScript
- Default function parameter values in JavaScript
- What's the Difference Between the Assignment (`=`) and Equality (`==`, `===`) Operators?
- Difference between `let` and `var` in JavaScript
- Difference between `var functionName = function() {}` and `functionName() {}` in JavaScript
- Disable / enable an input with jQuery?
- Find object by property in JavaScript array
- Get Selected Value in Dropdown List Using JavaScript
- Get the Current URL with JavaScript?
- Get the last item in an array using JavaScript
- Hide scroll bar, while still being able to scroll
- How are parameters sent in an HTTP POST request?
- How Can I Add a Key-Value Pair to a JavaScript Object?
- How can I change an element's class with JavaScript?
- How can I Check for "undefined" in JavaScript?
- How can I convert a string to a boolean in JavaScript?
- How can I create a two-dimensional array in JavaScript?
- How can I know which radio button is selected via jQuery?
- How do I Replace all Occurrences of a String in JavaScript?
- How do I remove/chop/slice/trim off the last character in a string using Javascript?
- How do I scroll to an element using jQuery?
- How do I Test for an Empty JavaScript Object?
- How do you append something to an array using JavaScript?
- How to add a class to an HTML element with JavaScript
- How to check if a key exists in a JavaScript object
- How to compare two dates with JavaScript
- How to find the sum of an array of numbers
- How to format numbers as currency strings
- How to get the length of a JavaScript object
- How to get values from URLs in JavaScript
- How to insert an item into an array at a specific index using JavaScript
- How to submit an AJAX form using jQuery
- HTTP GET request in JavaScript?
- JavaScript check if variable exists (is defined/initialized)
- How do JavaScript closures work?
- What does `"use strict"` do in JavaScript, and what is the reasoning behind it?
- Loop over an array in JavaScript
- Map function for objects (instead of arrays)
- Naming variables in JavaScript
- Parsing a string to a `Date` in JavaScript
- Redirect to Another Page Using JavaScript
- How Can I Remove a Specific Item from an Array?
- Removing Properties from Objects in JavaScript
- Return a value from an asynchronous function call
- Saving Arrays and Objects to Browser Storage Using JavaScript
- What is "Script Error"?
- Semicolon usage in JavaScript
- Set the select option 'selected' by value using jQuery
- Setting "checked" for a checkbox with jQuery
- Split a string in JavaScript
- How to Check Whether a String Contains a Substring in JavaScript?
- Truthy and falsy values in JavaScript
- Uncaught Error: Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function but got: object
- Uncaught ReferenceError: $ is not defined?
- Undefined versus null in JavaScript
- Warning: Each Child in a List Should Have a Unique 'key' Prop
- What does "javascript:void(0)" mean?
- What is the difference between POST and PUT in HTTP?
- What is the JavaScript Version of `sleep()`?
- What’s the difference between the window `load` event and the document `DOMContentLoaded` event?
- Which href value should I use for empty JavaScript links, "#" or "javascript:void(0)"?
- Why does my JavaScript code receive a "No Access-Control-Allow-Origin header is present on the requested resource" error, while Postman does not?
- Wrong References to `this`
- What is the `--save` option for npm install?
- How do I pass command line arguments to a Node.js program?
- Accessing Command Line Arguments
- Debug a Node.js server application
- Difference between NPM and NPX in JavaScript
- How do I resolve a "Cannot find module" error using Node.js?
- `module.exports` in Node.js
- What's the Difference Between Tilde (~) and Caret (^) in a `package.json` file?
- What's the difference between `dependencies`, `devDependencies`, and `peerDependencies` in npm `package.json` file?
- Updating Dependencies in `package.json`
- Update Node.js and NPM
- Find the version of an installed npm package
- Problems When Using 'for-in' with Arrays
- Check if a list is empty in Python
- Count occurrences of item in Python list
- How do I append one string to another in Python?
- How do I parse a string to a float or int?
- Access environment variables in Python
- Accessing the Index in a `for` Loop in Python
- Access the index of a for loop
- Add a new column to a DataFrame in Python Pandas
- Append vs. extend in Python
- Change the size of figures drawn with Matplotlib in Python
- Check for NaN values in Python
- Check if a dictionary contains a key in Python
- Check if a string is empty in Python
- Concatenate two lists in Python
- Convert a list to a string in Python
- Convert a string to bytes in Python
- Convert bytes to a string in Python
- Convert an integer to a string in Python
- Convert a string containing a date into datetime in Python
- Copy files in Python
- Declare an array in Python
- Define a two-dimensional array in Python
- Delete a column from a DataFrame in Python Pandas
- Delete a file or folder in Python
- Delete element from a dictionary in Python
- Determine the type of a Python variable
- Difference between `@staticmethod` and `@classmethod` function decorators in Python
- Extract a substring from a string in Python
- Fastest way to check if a list contains an item in Python
- How to manage feature flags without making performance worse
- Find all files in a directory with a given extension using Python
- Find item in list in Python
- Find the current directory and executed file's directory in Python
- Generate random integers within a range in Python
- Get key by value in dictionary Python
- Get the current script file's full directory path in Python
- Get the current time in Python
- Get the last element of a list in Python
- Get the length of a list in Python
- Get unique values from a list in Python
- How do I remove a trailing newline from a string in Python
- What does `if __name__ == "__main__":` do?
- Import files from a different folder in Python
- Indentation errors in Python
- Install a specific version of a Python package using PIP
- Install Python packages from a local package index
- Invalid literal for int error in Python
- Iterate over rows in a Pandas DataFrame in Python
- Limit floats to two decimal points in Python
- Is There a List of pytz Timezones?
- Logical AND (&&) in Python
- Lowercase a string in Python
- Making a list out of a list of lists in Python?
- Measure elapsed time in Python
- Pandas DataFrame row count in Python
- Print multiple times without newlines or spaces in Python
- Add new keys to a dictionary in Python
- Sort a dictionary by value in Python
- Python equivalent of a ternary operator: if-then-else in one line
- Iterate over a dictionary in Python
- List files in a directory using Python
- How Do I Merge Two Dictionaries in a Single Expression (Take Union of Dictionaries)?
- How Can I Safely Create a Nested Directory?
- Python slice notation
- Does Python Have a Ternary Conditional Operator?
- What Does the `yield` Keyword in Python Do?
- Raise an exception in Python
- Read a file line by line into a list Python
- Remove element from a list by index in Python
- Remove whitespace from a string in Python
- Rename columns in a Python Pandas DataFrame
- Running External Programs in Python
- Save a Matplotlib plot to a file in Python
- Select multiple columns in Python Pandas
- Select rows from a Python Pandas DataFrame based on column values
- Slicing in Python
- Split statements across multiple lines in Python code
- TypeError 'str' Object Does Not Support Item Assignment
- `super()` and `__init__()` in Python
- Switch-case statement equivalents in Python
- Time delay in Python
- UnboundLocalError Local Variable 'index' Referenced Before Assignment
- Upgrade specific `pip` packages in Python
- Use global variables in Python functions
- What are Python Metaclasses?
- Write one or more lines to a file in Python
- Can you force a React component to re-render without calling setState?
- How do I Conditionally add Attributes to React Components?
- Using Class Functions in Props for Components
- Encountered Two Children with the Same Key — React Fix
- setState is an Asynchronous Function
- How do you show or hide elements in React?
- Modifying State of a Component Directly
- Component State Should Always be Initialized Fully
- How do you loop inside React JSX?
- How Do You Pass Props to `{this.props.children}`?
- Programmatic navigation using React Router
- What Do These Three Dots `...` in React Do?
- Avoid re-rendering by using uncontrolled inputs
- Understanding unique keys for array children in React.js
- Using Props to Initialize State of Component
- What is the difference between React Native and React?
- Why can't the React.js onClick event pass a value to a method?
- Why don't React Router URLs work when refreshing or writing manually?
- Document/Window Object Error
- Auto-reloading a Flask application
- Difference Between `json.dumps()` and `flask.jsonify()`
- How Do You Get POSTed JSON in Flask?
- How do I Configure Flask Dev Server to be Visible Across the Network?
- How can I get the named parameters from a URL using Flask?
- How do you access the query string in Flask routes?
- How do you get the Data Received in a Flask Request?
- Redirect to a URL in Flask
- How Do I Return a JSON Response from a Flask View?
- How Do You Serve Static Files in Flask?
- ValueError: View Function Did Not Return a Response
- RuntimeError: Working Outside of Application Context
- How Can I Combine Two or More QuerySets in a Django View?
- Debugging a Django application
- What's the Difference Between `null=True` and `blank=True` in Django?
- What Does `on_delete` do in Django Models?
- How Do I Exclude Values in Django QuerySet Filtering?
- How to Revert the Last Migration?
- How do I access parameters from the URL as part the HttpRequest object?
- How Can I Temporarily Disable a Foreign Key Constraint in MySQL?
- What is a "slug" in Django?
- What is related_name used for?
- Determine whether a directory exists in Bash
- Check that a string contains a substring in Bash
- Concatenate string variables in Bash
- Delete all lines containing a string from a file in Bash
- Determine whether a file exists or not in Bash
- Find files in current and all subdirectories in Bash
- How can I set a custom HTTP header using cURL
- POST JSON data using cURL
- Print a multi-line string in Bash
- Set Bash variable to output of command
- Sending Cookies with Curl
- Split string on delimiter in Bash
- What are the differences between HashMap and Hashtable in Java?
- How do I convert a String to an int in Java?
- How do I declare and initialize an array in Java?
- Create ArrayList from array in Java
- Avoiding `NullPointerException` in Java
- Why is a `char[]` Preferred Over a String for Passwords?
- How can I create a memory leak in Java?
- How to compare strings in Java
- How to create a file and write to it in Java
- How to initialize an array in one line in Java
- How to split a string in Java
- How to Throw Exceptions in Java
- How do I Read / Convert an InputStream into a String in Java?
- How to Iterate Over a HashMap in Java
- Why don't Java's compound assignment operators require casting?
- How to initialize an array in Java?
- Is Java Pass-By-Reference or Pass-By-Value?
- How to sort a List in Java?
- How to Lexicographically Sort an Array in Java
- How do I generate random integers within a specific range in Java?
- How to Read a File in Java
- What does "Could not find or load main class" mean?
- What is a NullPointerException, and how do I fix it?
- What is the difference between public, protected, package-private and private in Java?
- How to Write to a File in Java
- How to change the CSS background opacity of an element
- How do I auto-resize an image to fit a "div" container?
- How do I Create an HTML Button That Acts Like a Link?
- How can I set the default value for an HTML select element?
- How do I add a tab space instead of multiple non-breaking spaces ("nbsp")?
- How do I vertically center text with CSS?
- How to align a div to the middle (horizontally / width) of the page?
- How to align content of a div to the bottom
- How to change the cursor into a hand when a user hovers over a list item (CSS)
- How to disable text selection highlighting (css)
- How to make a div 100% height of the browser window?
- How to open a URL in a new tab (and not a new window)
- How do I overlay one div over another div?
- How to remove focus border (outline) around text/input boxes
- How to style a checkbox using CSS
- HTML text input - allow only numeric input
- I need an unordered list without any bullets (HTML, CSS)
- Is there a CSS parent selector?
- Regex to match open HTML tags except for self-contained tags
- Abort a merge in Git
- Change the URI for a Git remote
- Check out a remote branch in Git
- Clone a Git repository to a specific folder
- Clone a specific Git repository branch
- Create a new Git branch from an existing branch
- Create a remote branch in Git
- Delete a commit from a branch in Git
- Delete a file from a Git repository
- Delete a Git branch locally and remotely
- Determine the origin of a cloned Git repository
- Discard unstaged changes in Git
- Fetch a remote branch in Git
- Fetch and pull all branches in Git
- Fix "detached HEAD" in Git
- Force `git pull` to overwrite local files
- Git "failed to push some refs to remote"
- Git src refspec master does not match any error message
- Merge branches with unrelated histories in Git
- Modify an existing unpushed commit message in Git
- Push a local branch to a remote repository in Git
- Pulling vs. fetching in Git
- Remove untracked files from the working tree in Git
- Rename a local Git branch
- Reset a local branch to remote state in Git
- Resolve merge conflicts in Git
- Revert a Git repository to a previous commit
- Revert local changes in Git repository to a previous state
- Revert single file to earlier state in Git
- Save credentials for remotes in Git
- Show the current branch name in Git
- Squash commits together in Git
- Undo an unpushed Git merge
- Undo git add before commit
- Undo the most recent local Git commits
- View the change history of a single file in Git
- Apply permissions to a directory and its contents in Linux
- Change permissions for a folder and subfolders in Linux
- Copy folder with files to another folder in Linux
- Find all files containing a specific string
- How can I symlink a file in Linux?
- How to Exit Vim
- SCP remote to local
- Write a regular expression to match lines not containing a word
- How do I find duplicate values in a SQL table?
- How do I import a SQL file using the command line in MySQL?
- How do I insert multiple rows in a single SQL query?
- How do I perform an IF...THEN in a SQL SELECT?
- How do I use OR in a CASE statement in SQL Server?
- How to SELECT in SQL where the field contains words?
- How do you INSERT INTO VALUES from a SELECT FROM in every database in SQL?