What is the difference between null and undefined in JavaScript?
Theme: JavaScript Role: Front End Developer Function: Technology
Interview Question for Front-End Developer: See sample answers, motivations & red flags for this common interview question. About Front-End Developer: Creates the user interface and user experience of websites and applications. This role falls within the Technology function of a firm. See other interview questions & further information for this role here
Sample Answer
Example response for question delving into JavaScript with the key points that need to be covered in an effective response. Customize this to your own experience with concrete examples and evidence
- Definition: Null is a value that represents the intentional absence of any object value. It is a primitive value in JavaScript. Undefined is a value that is automatically assigned to variables that have been declared but have not been assigned a value yet
- Type: Null is of type object. Undefined is of type undefined
- Assignment: Null can be assigned to a variable explicitly by the programmer. Undefined is automatically assigned to variables that have not been assigned a value
- Usage: Null is often used to indicate the absence of an object or value. Undefined is commonly used to check if a variable has been assigned a value or not
- Equality: Null is equal to null and undefined is equal to undefined, but they are not equal to each other
- Conversion: Null can be converted to a number, which results in 0. Undefined, when converted to a number, results in NaN (Not a Number)
- Object Properties: Null can have properties and methods assigned to it. Undefined cannot have properties or methods assigned to it
- Function Parameters: Null can be passed as an argument to a function. Undefined is often used as the default value for function parameters
- Error Handling: Null is considered a valid value and does not cause an error. Undefined can cause errors when trying to access properties or methods of an undefined variable
Underlying Motivations
What the Interviewer is trying to find out about you and your experiences through this question
- Technical knowledge: Assessing the candidate's understanding of fundamental JavaScript concepts
- Problem-solving skills: Evaluating the candidate's ability to differentiate between different types of values and handle them appropriately
- Attention to detail: Checking if the candidate pays attention to small nuances and understands the subtle differences between null and undefined
Potential Minefields
How to avoid some common minefields when answering this question in order to not raise any red flags
- Confusing null & undefined: Mixing up the concepts and not clearly explaining the difference between null and undefined
- Lack of technical knowledge: Not being able to provide a clear and accurate explanation of the differences between null and undefined in JavaScript
- Inability to give examples: Failing to provide concrete examples or use cases to illustrate the differences between null and undefined
- Overcomplicating the answer: Providing a convoluted or overly complex explanation instead of keeping it simple and concise
- Lack of understanding of JavaScript fundamentals: Showing a lack of understanding of basic JavaScript concepts and how null and undefined are used in the language