02. Let and Const

There are now two new ways to declare variables in JavaScript: let and const.

Up until now, the only way to declare a variable in JavaScript was to use the keyword var. To understand why let and const were added, it’s probably best to look at an example of when using var can get us into trouble.

Take a look at the following code.

Hoisted to the Top

What do you expect to be the output from running getClothing(false)?

function getClothing(isCold) {
  if (isCold) {
    var freezing = 'Grab a jacket!';
  } else {
    var hot = 'It’s a shorts kind of day.';
    console.log(freezing);
  }
}
SOLUTION: undefined

Hoisting

Hoisting is a result of how JavaScript is interpreted by your browser. Essentially, before any JavaScript code is executed, all variables are "hoisted", which means they're raised to the top of the function scope. So at run-time, the getClothing() function actually looks more like this…

Hoisting Animation

INSTRUCTOR NOTE:

Before the function is executed, all variables are hoisted to the top of the function scope. So what’s our solution?

let and const

Variables declared with let and const eliminate this specific issue of hoisting because they’re scoped to the block, not to the function. Previously, when you used var, variables were either scoped globally or locally to an entire function scope.

If a variable is declared using let or const inside a block of code (denoted by curly braces { }), then the variable is stuck in what is known as the temporal dead zone until the variable’s declaration is processed. This behavior prevents variables from being accessed only until after they’ve been declared.

Block Scoped

INSTRUCTOR NOTE:

Variables declared with `let` and `const` are only available within the block they're declared.

Temporal Dead Zone

What do you expect to be the output from running getClothing(false)?

function getClothing(isCold) {
  if (isCold) {
    const freezing = 'Grab a jacket!';
  } else {
    const hot = 'It’s a shorts kind of day.';
    console.log(freezing);
  }
}
SOLUTION: ReferenceError: freezing is not defined

Rules for using let and const

let and const also have some other interesting properties.

  • Variables declared with let can be reassigned, but can’t be redeclared in the same scope.
  • Variables declared with const must be assigned an initial value, but can’t be redeclared in the same scope, and can’t be reassigned.

Assignment with Let

What do you expect to be output from running the following code?

let instructor = 'James';
instructor = 'Richard';
console.log(instructor);
SOLUTION: Richard

Use cases

The big question is when should you use let and const? The general rule of thumb is as follows:

  • use let when you plan to reassign new values to a variable, and
  • use const when you don’t plan on reassigning new values to a variable.

Since const is the strictest way to declare a variable, we suggest that you always declare variables with const because it'll make your code easier to reason about since you know the identifiers won't change throughout the lifetime of your program. If you find that you need to update a variable or change it, then go back and switch it from const to let.

That’s pretty straightforward, right? But what about var?

What about var?

Is there any reason to use var anymore? Not really.

There are some arguments that can be made for using var in situations where you want to globally define variables, but this is often considered bad practice and should be avoided. From now on, we suggest ditching var in place of using let and const.

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