In JavaScript, undefined is a primitive value that is automatically assigned to variables that have been declared but not yet initialized with a value. It’s also the default return value of functions that do not explicitly return anything.
Here are some key points about undefined:
- It represents the absence of a value.
- It’s a primitive data type.
- It’s automatically assigned to variables that have been declared but not yet assigned a value.
- It’s returned when accessing properties of an object that do not exist.
-
It’s considered a falsy value, meaning it evaluates to
falsein a Boolean context.
Example: Here, the variable x is declared but not assigned a value, so it automatically gets the value undefined. In the second example, the function greet() is called without passing any arguments, so the parameter name inside the function has the value undefined.
Javascript
let x;console.log(x); // Output: undefined
function greet(name) {
console.log(`Hello, ${name}!`);
}greet(); // Output: Hello, undefined!
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Output
undefined Hello, undefined!