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JS Cheatsheet

Published on August 1, 2021

Var, Let and Const#

Javascript ES6 introduced two new keywords to define variables, let and const. Previously, the keyword var was the only way to do so. let and const were introduced because there were issues with var which made it error-prone and hard to debug.

varletconst
ScopeGlobal or FunctionBlock scope {}Block scope {}
Must be initialized when declaredNoNoYes
Can use before initializationYesNoNo
Can be redeclaredYesNoNo
Can be updatedYesYesNo
Hoisted to start of scopeYesYesYes

Avoid using var because it is either global or function scope, and a declaration and assignment of a var can easily be redeclared or updated unknowingly in another function. Furthermore, a var is initialized with undefined and can be used even before being assigned a value.

On the other hand, let and const are block scope, and cannot be redeclared. Additionally, const cannot be updated. Both let and const also cannot be used before initialization, and will throw Reference error rather than return undefined like var.

Quote

"Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution." - Sarah Chima Atuonwu, Var, Let, and Const – What's the Difference?

Resources#