![]() ![]() While var and let can be declared without being initialized, const must be initialized during declaration.But while var variables are initialized with undefined, let and const variables are not initialized. They are all hoisted to the top of their scope.var variables can be updated and re-declared within its scope let variables can be updated but not re-declared const variables can neither be updated nor re-declared.var declarations are globally scoped or function scoped while let and const are block scoped.So just in case you missed the differences, here they are: Just like let, const declarations are hoisted to the top but are not initialized. This will update the value of ssage without returning errors. We can do this: ssage = "say Hello instead" let is block scopedĪ block is a chunk of code bounded by // error: Assignment to constant variable. It also solves the problem with var that we just covered. It's no surprise as it comes as an improvement to var declarations. Let is now preferred for variable declaration. This will likely cause a lot of bugs in your code. ![]() If you have used greeter in other parts of your code, you might be surprised at the output you might get. While this is not a problem if you knowingly want greeter to be redefined, it becomes a problem when you do not realize that a variable greeter has already been defined before. So, since times > 3 returns true, greeter is redefined to "say Hello instead". I'll use the example below to explain: var greeter = "hey hi" Ĭonsole.log(greeter) // "say Hello instead" So var variables are hoisted to the top of their scope and initialized with a value of undefined. This means that if we do this: console.log (greeter) Ĭonsole.log(greeter) // greeter is undefined Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. This means that we can do this within the same scope and won't get an error. var variables can be re-declared and updated We'll get an error which is as a result of hello not being available outside the function. So if we do this: var tester = "hey hi" Ĭonsole.log(hello) // error: hello is not defined So we cannot access the variable hello outside of a function. Here, greeter is globally scoped because it exists outside a function while hello is function scoped. To understand further, look at the example below. This means that it is available and can be accessed only within that function. Var is function scoped when it is declared within a function. This means that any variable that is declared with var outside a function block is available for use in the whole window. The scope is global when a var variable is declared outside a function. var declarations are globally scoped or function/locally scoped. Scope essentially means where these variables are available for use. First, let's get to understand var more before we discuss those issues. That is why it was necessary for new ways to declare variables to emerge. There are issues associated with variables declared with var, though. Here's an Interactive Scrim of Var, Let, and Const Varīefore the advent of ES6, var declarations ruled. As you read, take note of the differences between them that I'll point out. In this article, we'll discuss var, let and const with respect to their scope, use, and hoisting. The question is, what makes them different from good ol' var which we've been using? If you are still not clear about this, then this article is for you. One of the features that came with ES6 is the addition of let and const, which can be used for variable declaration. While this assumption might be partially true, it's still possible that some of these features remain a mystery to some devs. And now, since it's 2020, it's assumed that a lot of JavaScript developers have become familiar with and have started using these features. A lot of shiny new features came out with ES2015 (ES6). ![]()
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