JavaScript developers need to know the difference between 'undefined' and 'null'. Learn what these two mean in JavaScript and how they are different from each other. After that, we will also share some tips on how to handle them properly so that you find them easier to use.
After reading this article till the end, you will have a clear understanding of what the difference is between undefined and void in JavaScript and when and how to use 'undefined' and 'void' effectively in your JavaScript code.
So let's get started.
Table of contents
- What is undefined in JavaScript?
- What is a null in JavaScript?
- What is the difference between undefined and null in JavaScript?
- Handling undefined and null
- Best practices for undefined and null
What is undefined in JavaScript?
Undefined is a primitive value in JavaScript. It's automatically assigned to variables that have been declared but not yet initialized with a value. It indicates that a variable exists in the current scope but has not been assigned any specific value.
let variable;
console.log(variable); // Output: undefinedHere, a variable is declared but not assigned any value, resulting in the output being undefined.
What is null in JavaScript?
In JavaScript, Null is a primitive data type. It's used to represent the intentional absence of any value or object. Programmers often assign null to a variable to denote that the variable is declared and initialized and currently doesn’t hold a value. Still, a value can be expected in the future.
let nullVariable = null;
console.log(nullVariable); // Output: nullIn this example, nullVariable is explicitly assigned the value null, indicating the absence of a value.
What is the difference between undefined and null in JavaScript?
Here, let's explore the differences between undefined and null one by one:
#1. Comparison of Null and Undefined
Null and undefined may seem similar at first look, but it's important to understand that they are not exactly the same in JavaScript.
console.log(null == undefined); // true
console.log(null === undefined); // false#2. Types of Null and Undefined
In JavaScript, it is important to understand that undefined is a type, shown by typeof undefined returning 'undefined'. whereas, null is considered to be an object, as indicated by typeof null returning 'object'.
console.log(typeof undefined); // undefied
console.log(typeof null); // object#3. Assignment
When something doesn't have a value, undefined is automatically chosen as "undefined." But here's the thing: JavaScript won't ever give something the value "null" on its own. Only if a programmer or a piece of code specifically says, "Hey, this thing is null," will it actually be null.
So, while JavaScript uses "undefined" by default for empty things, "null" only shows up if we make it happen ourselves.
let variable1; // variable1 is automatically assigned as undefined
console.log(variable1); // Output: undefined
let variable2 = null; // variable2 is explicitly assigned as null
console.log(variable2); // Output: null#4. Handling Arithmetic Operations
During arithmetic operations in JavaScript, null is converted to 0, while undefined is converted to NaN (Not a Number).
This conversion happens because JavaScript internally calls the toNumber() function, which translates null to 0 and undefined to NaN before operating.
For example When null is involved in arithmetic operations
let num = null;
console.log(num + 5); // Output: 5
console.log(num * 10); // Output: 0For example When undefined is involved in arithmetic operations
let num;
console.log(num + 5); // Output: NaN
console.log(num * 10); // Output: NaNwhen performing arithmetic operations, null behaves like 0, while undefined results in NaN. It's important to handle these scenarios appropriately in your code to avoid unexpected behavior.
#5. Implicit vs. Explicit Assignment
When no value is explicitly assigned to a variable during its declaration, it naturally assumes the status of being undefined. This occurs because the variable exists in the program's scope but has no defined value.
When we purposely assign the null value to a variable using the assignment operator, it is immediately identified as having a null value. This deliberate behavior indicates a shortage of a meaningful value or object reference.
// Implicit Assignment to Undefined
let variable1; // variable1 is implicitly assigned as undefined
console.log(variable1); // Output: undefined
// Explicit Assignment to Null
let variable2 = null; // variable2 is explicitly assigned as null
console.log(variable2); // Output: null In JavaScript, null and undefined behave differently. Knowing these differences is important for writing dependable code. Null is used to say there's no value, while undefined means a variable doesn't have a value yet.
Null VS Undefined
| Aspect | null | undefined |
|---|---|---|
| Definition | Represents the intentional absence of value or object. | Automatically assigned to variables that have been declared but not yet initialized with a value. |
| Assignment | Must be specially assigned. | Automatically assigned if a value is not assigned. |
| Type | Null is a Primitive data type that represents the null, empty, or non-existent reference. | Undefined is a primitive value that is used when a variable has not been assigned a value. |
| Typeof | Typeof null returns 'object' |
Typeof undefined returns 'undefined'. |
| Comparison | "null == undefined" returns true. | "null === undefined" returns false. |
| Arithmetic Operations | Null is converted to zero (0) while performing primitive operations. | Undefined is converted to NaN while performing primitive operations. |
| Usage | Used to signify the absence of a value or object reference. | Indicates that a variable has not been assigned a value. |
| Initialization | Can be assigned to a variable to denote no value. | Automatically assigned to uninitialized variables. |
| Causes | Explicitly assigned when necessary. | Generally occurs due to variable declaration without initialization. |
Handling Undefined and null
In JavaScript, we often come across situations where variables may be undefined or null. It's important to deal with these situations correctly to make sure our code works well and doesn't have any bugs. Here are some ways to check for and deal with both undefined and null variables:
#1. Checking for Undefined:
To check if a variable is undefined, you can use the typeof operator:
// Define a variable with an undefined value
let undefinedVariable;
// Check if the variable is undefined
if (typeof undefinedVariable === 'undefined') {
console.log('The variable is undefined.'); // Output: The variable is undefined.
} else {
console.log('The variable is defined.');
}In this code example, we define a variable undefinedVariable without assigning any value to it, resulting in it being undefined. It then checks if the variable is undefined using typeof operator.
#2. Checking for Null:
To check if a variable is null, use direct comparison.
// Define a variable with a null value
let nullVariable = null;
// Check if the variable is null
if (nullVariable === null) {
console.log('The variable is null.'); // Output: The variable is null.
} else {
console.log('The variable is not null.');
}In the above example, we define a variable nullVariable and explicitly assign it the value null. It then checks if the variable is null using a direct comparison.
Properly handling both undefined and null ensures that your JavaScript code behaves predictably and prevents potential errors caused by unexpected values.
Best Practices for undefined and null
Properly managing undefined and null in JavaScript is important for writing clean and reliable code. Below are some best practices to follow:
#1. Initialize Variables with Meaningful Values:
undefined:
Always initialize variables with meaningful values to avoid confusion between undefined and null. This ensures that variables have defined values from the start.
Variable Initialization: Use undefined to initialize variables that may not have a value immediately but will be assigned one later in the code.
let username = undefined;
// Later in the code
username = "Elena";Function Return Values: Return undefined explicitly from functions to indicate that the function does not produce a meaningful value.
function findUser(id) {
if (id === '') {
return undefined;
}
// Function logic
}null:
Explicitly use 'null' to indicate the absence of a value or reset a variable to no value.
Explicit Initialization: Use null to explicitly indicate the absence of a value or reset a variable to no value.
let currentUser = null;
// Later in the code
currentUser = findUser(id);Resetting Object References: Use null to reset object references when they are no longer needed or to clear memory.
let data = fetchData();
// Process data
data = null; // Clearing the object reference #2. Use Strict Equality (===) for Comparisons:
When comparing variables, use strict equality (===) to recognize between undefined and null. Strict equality checks both the value and the type, providing more accurate comparisons.
#3. Be Cautious with Null Usage:
Be careful when using null, as it can sometimes lead to unexpected behaviors. Think about whether you need to use null in your code, and make sure to use it correctly to avoid mistakes.
Conclusion
Any JavaScript developer needs to know the difference between 'undefined' and 'null'. Just remember that undefined' means a variable has been declared but not assigned a value and 'null' means a variable intentionally has no value. By understanding this you will be able to handle 'undefined' and 'null' in JavaScript like a pro. Keep practicing and improving your skills to become even better at evolution!
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