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let, hoisting, and the temporal dead zone with vanilla JS

Yesterday, we looked at how variables declared with let treat scope a bit differently than variables declared with var. Today, we’re going to look at how they treat hoisting different.

If you don’t know what hoisting is yet, start with this article on variable hoisting in JavaScript.

Traditional hoisting with var

When a JavaScript file first loads, the browser does any initial pass and stores any variable names it finds to memory. This is called hoisting.

When the browser goes to actually execute the file, it already knows what the variable names are, but does not know what their values are.

In the example below, v was hoisted. Trying to log its value before it’s declared logs undefined, but doesn’t throw an error. The browser knows it exists, but it gets a temporary value of undefined until the declaration happens.

// The variable name is hoisted, but the value is undefined until it's declared
// logs undefined

// The value gets defined
var v = 'declare with var';

// logs "declare with var"

Hoisting with let

Variables declared with let are still hoisted, but they don’t automatically receive a value of undefined the way variables declared with var do.

The browser knows they exist, but has no value at all for them. Trying to log a variable declared with let before it’s actually declared throws an error.

// While the name was hoisted, it has no defined value and throws an error
// Uncaught ReferenceError: l is not defined

// The value gets defined
let l = 'declare with let';

// logs "declare with var"

This gap between when a variable declared with let is hoisted and when it actually gets declared is known as the temporal dead zone.

Steve Griffith has a great video on this if you want to learn more.

This is actually a good thing

In yesterday’s article, I wrote that I like the more strict behavior that let introduces, and this is another example.

The let operator forces you to declare a variable before trying to use it, and I think that’s a good thing.