Scope of variables declared with var
Variables declared using the
var keyword exhibit function-scoping behavior, which is different from the block-scoping behavior of variables declared with
const. Function-scoped variables are accessible throughout the entire function in which they are declared, regardless of the block they are defined in. This can lead to unintended issues, especially when working with nested functions or blocks, as variables declared with
var might be accessible in places where they were not intended to be used.
In contrast, block-scoped variables declared with
const are only accessible within the block they are defined in, providing a more predictable and controlled environment for managing variable visibility and lifetime. This helps to reduce the likelihood of encountering bugs related to variable scope and makes the code easier to understand and maintain.
The second reason to avoid declaring variables with var is due to a phenomenon known as hoisting.
var can be accessed and manipulated before their actual declaration in the code, which can lead to unexpected results and make the code harder to understand and maintain.
In contrast, when using
const for variable declarations, hoisting still occurs, but the variables are not initialized until their actual declaration is reached in the code. This creates a temporal dead zone (TDZ) between the beginning of the scope and the point of declaration, during which accessing or manipulating the variables would result in a ReferenceError. This behavior helps to prevent unintended consequences and encourages a more predictable coding environment.
The last significant reason to avoid using
var can be re-declared without any warnings or error messages. This lack of feedback can result in unintentional overwriting of variables, which may lead to unexpected behavior in your code and create hard-to-find bugs.
When a variable is re-declared using
var, the original value is simply replaced with the new one, and the code continues to execute without any indication that something may have gone wrong. This can be particularly problematic in larger codebases or when working with multiple developers, as it increases the likelihood of accidentally overwriting a variable that was intended to remain constant throughout the code.
In contrast, using
const to declare variables provides a safeguard against this issue. If you attempt to re-declare a variable that has already been declared with
In summary, although the
The function-scoping behavior, hoisting peculiarities, and the possibility of re-declaration without warnings make
var a less predictable and potentially riskier choice.
On the other hand,
const provide block-scoping, controlled hoisting, and protection against re-declaration, fostering a more reliable and maintainable coding environment.
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This article is part of a series called Bit by Bit, a series devoted to all things programming. Whether you're still a computer science undergrad or the CTO of Apple, there's something for you here.
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