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JavaScript

Posted by Benjamin Hedgepeth
Benjamin Hedgepeth
5,672 Points

Variable Initialization in Javascript

I'm extremely confused with the initialization stage when it comes to variable statements

http://www.ecma-international.org/ecma-262/6.0/#sec-variable-statement

Var variables are created when their containing Lexical Environment is instantiated and are initialized to undefined when created.

Initialization is the means of assigning an initial value to a variable. So undefined is the initial value of a declared variable.

Seeing this though various times;

var thing = "code";

The variable thing is initialized with "code".

How is this possible? Initializing a variable that is already initialized does not make any sense to me at all. In other words, how can a variable be initialized when it already has an initial value?

1 Answer

Steven Parker
Steven Parker
229,732 Points
Steven Parker
Steven Parker
229,732 Points

In your example, the variable "thing" is being created for the first time.

If it already existed (initialized or just declared), there would be no "var" in front of the name and this would be a simple assignment.

Benjamin Hedgepeth
Benjamin Hedgepeth
5,672 Points

I understand that. Please watch this video:

https://www.youtube.com/watch?v=6vBYfLCE9-Q

He says a variable is initialized with a value of undefined. Then he goes on to say that same variable is initialized with a string.

From MDN:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var

The variable statement declares a variable, optionally initializing it to a value

How is it that there is an option to initialize a variable when it's automatically initialized with a default value by JavaScript when it is declared?

Steven Parker
Steven Parker
229,732 Points
Steven Parker
Steven Parker
229,732 Points

I think what he meant to say was, "Variables are implicitly initialized to a value of undefined by default when they are created." So that only happens if they are not initialized explicitly. For example:

var one;      // this variable is implicitly initialized with "undefined"
var two = 2;  // this variable is explicitly initialized with the numeric value 2

The reason there is an option to explicitly initialize a variable when you declare it is so you don't need to assign it in a separate step.