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
- Capturing JavaScript Errors
- How do I check if an element is hidden in jQuery?
- Warning: componentWillMount has been Renamed, and is not Recommended for Use
- 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
- Get Selected Value in Dropdown List Using JavaScript
- Get the Current URL with JavaScript?
- Get the last item in an array using JavaScript
- 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 convert a string to a boolean in JavaScript?
- How can I Check for "undefined" 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 Empty an Array in JavaScript?
- 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 do I Replace all Occurrences of a String in JavaScript?
- How do I Test for an Empty JavaScript Object?
- How do you append something to an array using JavaScript?
- How to check if a key exists in a JavaScript object
- How to compare two dates with JavaScript
- How to insert an item into an array at a specific index using JavaScript
- 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
- Naming variables 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
- 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
- Undefined versus null in JavaScript
- Warning: Each Child in a List Should Have a Unique 'key' Prop
- What is the JavaScript Version of `sleep()`?
- 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`
- 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
- `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?
- What is the `--save` option for npm install?
- 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
- Accessing the Index in a `for` Loop in Python
- Difference between `@staticmethod` and `@classmethod` function decorators in Python
- How to manage feature flags without making performance worse
- How do I parse a string to a float or int?
- What does `if __name__ == "__main__":` do?
- Is There a List of pytz Timezones?
- Making a list out of a list of lists in Python?
- Add new keys to a dictionary in Python
- Sort a dictionary by value in Python
- 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?
- Running External Programs in Python
- TypeError 'str' Object Does Not Support Item Assignment
- UnboundLocalError Local Variable 'index' Referenced Before Assignment
- Upgrade specific `pip` packages in Python
- What are Python Metaclasses?
- Using Class Functions in Props for Components
- Can you force a React component to re-render without calling setState?
- Encountered Two Children with the Same Key — React Fix
- setState is an Asynchronous Function
- How do I Conditionally add Attributes to React Components?
- 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?
- Auto-reloading a Flask application
- Difference Between `json.dumps()` and `flask.jsonify()`
- How do I Configure Flask Dev Server to be Visible Across the Network?
- How Do You Get POSTed JSON in Flask?
- 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?
- Create ArrayList from array in Java
- Avoiding `NullPointerException` in Java
- Why is a `char[]` Preferred Over a String for Passwords?
- What are the differences between HashMap and Hashtable in Java?
- How can I create a memory leak 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 is the difference between public, protected, package-private and private in Java?
- How to Write to a File in Java