Thread by @swapnakpanda

Let's redefine the "undefined". Can we do it in JavaScript?

⇩

➊ What is "undefined"?

❑ "undefined" is a variable in the global scope. The value of "undefined" is the primitive value "undefined"

❒ When a variable is declared but not assigned any value, value of global variable "undefined" is assigned to it implicitly.

➋ "undefined" is not a reserved keyword

❑ It means you can declare a variable named "undefined".

❒ So, what will happen if you declare a variable called "undefined" and assign some other value to it?

Let's explore.

➌ Redefining "undefined": Part 1

❑ We already said, "undefined" is a variable in the global scope.

❒ So, you can't declare another variable named "undefined" in the global scope.

➍ Redefining "undefined": Part 2

❑ You can declare a variable named "undefined" in a function or, block. That's allowed.

❒ And as you might have suspected, we lost access to "undefined" in global scope.

❑ You can access it using "window.undefined".

💭 Final Words

❑ "undefined" is not a reserved keyword. Hence you can redefine it, but not in global scope.

❒ Though it's allowed, we should avoid doing it. It will pollute the code. Consider this as a BAD PRACTICE.

End of 🧵

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