What is the difference between var, let, and const?
What is the difference between var, let, and const?
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Updated 20 Mar 2025
In JavaScript,
var,let, andconstare used to declare variables, but they have key differences in terms of scope, hoisting, and mutability.The
varkeyword has been used since the early days of JavaScript and is function-scoped, meaning it is only accessible within the function where it is declared. However, if declared outside a function, it becomes a global variable.varis hoisted, which means the variable is moved to the top of its scope at runtime but remains undefined until it is assigned a value. One major drawback ofvaris that it can be redeclared and updated, which can lead to unintended bugs, especially in large codebases.The
letkeyword was introduced in ES6 and provides block-level scope, meaning it is only accessible within the block{}where it is declared. Unlikevar,letis not hoisted in a way that allows usage before declaration, preventing issues caused by undefined variables. Additionally,letallows reassignment of values but does not allow redeclaration within the same scope, making it a safer and more predictable choice compared tovar.The
constkeyword also has block-level scope, similar tolet, but it differs in that variables declared withconstcannot be reassigned. This makesconstideal for declaring values that should remain constant throughout the program, such as configuration settings or fixed values. However,constdoes not make objects or arrays immutable; it only prevents reassigning the variable itself. The properties of an object or elements of an array declared withconstcan still be modified.Overall,
letandconstare preferred overvarin modern JavaScript due to their improved scoping and predictability.letshould be used when a variable’s value needs to change, whileconstshould be used for values that should remain unchanged.