blockscoped

Blockscoped, in the context of programming, refers to a scoping model where bindings are limited to the innermost block that contains them. In JavaScript, block-scoped declarations are created with let and const, introduced in ECMAScript 2015. Unlike var, which is function-scoped, let and const are confined to the block enclosed by curly braces, such as the body of an if statement, a loop, or a try/catch block. This behavior helps prevent variables from leaking into outer scopes and reduces certain categories of bugs.

Key characteristics of block-scoped bindings include the temporal dead zone and different hoisting behavior. A let or const binding is hoisted to the top of its block but remains uninitialized until the corresponding declaration is executed, meaning that accessing it before initialization results in a ReferenceError. This contrasts with var, which is hoisted and initialized with undefined, potentially leading to subtle errors. In for loops, using let for the loop index creates a separate binding for each iteration, which affects how closures capture values inside the loop.

Block scope is a widely used concept beyond JavaScript, and many other languages provide block-level scoping, including C, C++, Java, and Rust. The term block-scoped is commonly used to distinguish block-level scope from function-level or global scope. In JavaScript ecosystems, block-scoped behavior is supported by modern runtimes and is preserved by TypeScript and various transpilers when targeting contemporary JavaScript editions.