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What is undefined in JavaScript?


What is undefined in JavaScript?

In JavaScript, undefined is a primitive value that represents the absence of a value. It is one of the two falsy values in JavaScript, the other being null.

undefined is assigned to a variable when that variable has not been assigned a value yet, or when a function is called without any arguments.

How to check if a value is undefined

You can use the typeof operator to check if a value is undefined.


const x = undefined;

if (typeof x === "undefined") {
  console.log("x is undefined");
} else {
  console.log("x is not undefined");
}

The above code will log “x is undefined” to the console.

When to use undefined

undefined is typically used to indicate that a variable has not been assigned a value yet.


let x; // x is undefined

x = 10; // x is now 10

undefined can also be used as a default value for function parameters.


function greet(name) {
  name = name || "World"; // if name is undefined, set it to "World"

  console.log("Hello, " + name + "!");
}

greet(); // logs "Hello, World!"

Conclusion

undefined is a useful value in JavaScript that can be used to indicate the absence of a value. It is important to understand how undefined works in order to write correct and efficient JavaScript code.

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