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Understanding Global Variables in JavaScript

Understanding Global Variables in JavaScript
Reading time: 3 min
Hostman Team
Technical writer

Global variables in JavaScript are essential to programming as it offers accessibility throughout a script’s many sections. These are the variables that are defined outside of all functions and in the main body of the source code. Understanding what they are and how to use them is vital for developing robust applications.

Declaring and Using Global Variables

Global variables can be declared by simply declaring them in the main body of the source code and outside of any block or function. They can be declared by var, let, and const keywords. 

The var keyword is the oldest way to declare a variable. Variables declared outside of a function can be accessed globally, while variables declared inside a specific function can be accessed within that function. This feature is known as global scoped or function scoped.  

Example 1: In this example, the variable was declared at the start of the program outside every function using the var keyword.

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If the value was assigned to a variable but the variable was not declared, it will automatically be considered a global variable. 

Example 2: In this example, variable GlobalVariable was assigned but not declared, hence it will automatically become a global variable.  This example also shows that you may change the value of the global variable outside the function.

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The let and const keywords can also be used in declaring global variables in JS. 

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Note: Variables declared using let can have their values changed, whereas constants declared with const cannot be reassigned. 




How to Use Global Variables Effectively

It's important to pay attention to best practices while using global variables in JavaScript in order to reduce potential problems and encourage maintainability. Here's how to make efficient use of global variables.

  • Do not overuse. Limit the amount of time spent using global variables by enclosing data in functions or objects. This increases code maintainability, encourages modularity, and lowers the possibility of naming disputes.

  • To prevent naming conflicts, group global variables under a single namespace object.

  • Ensure that global variables are properly set up before using them elsewhere by initializing them at the start of the script or inside an initialization function.

  • It is a best practice to use the keywords var, let, or const when declaring global variables to avoid accidental generation of implicit global variables, which can result to difficult to find problems. 


In conclusion, coding is similar to learning how to balance. You must strike a balance between solidity and flexibility, simplicity and complexity. Like other programming tools, global variables have advantages and disadvantages. By enabling universal data access, they can simplify things. But if not handled with caution, they can potentially cause havoc.