How Does `let` Affect For Loop Behavior and Variable Scoping in JavaScript?

Understanding let and Block Scoping with For Loops

let prevents duplicate declarations and allows variables to be used within closures. However, its application to for loops can be somewhat perplexing. In this context, let encloses each iteration within a new lexical environment, binding the loop variable to its own distinct scope.

Consider the following code:

// prints '10' 10 times
for (var i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }

// prints '0' through '9'
for (let i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }

In the first loop, var is used, which creates a global variable. Consequently, each iteration of the loop accesses the same global variable i, resulting in the output '10' ten times.

In the second loop, let is employed, creating a new block scope for each iteration. This means that each iteration has its own distinct i variable, isolated from the others. Hence, the output displays '0' through '9'.

This mechanism is more than mere syntactic sugar. It involves creating a unique lexical environment for each iteration, safeguarding the loop variable within its exclusive scope.

To illustrate, the "desugared" code for the var loop expands to a simple linear sequence of steps. In contrast, the "desugared" code for the let loop introduces brace-enclosed blocks to explicitly demarcate each iteration's scope, reflecting the distinct lexical environment for each iteration.

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