Let

The let keyword was introduced in ES6 (2015). It is the modern way to declare variables that can be reassigned. Variables defined with let have several key characteristics that make them safer than var.

Before ES6, JavaScript only had Global Scope and Function Scope. let introduces Block Scope. Variables declared inside a { } block cannot be accessed from outside the block:

By contrast, variables declared with var do not have block scope:

Variables defined with let cannot be redeclared in the same scope. This prevents accidental overwriting of variable values.

With var, you can redeclare variables anywhere, which often leads to bugs:

Variables defined with let are also hoisted to the top of the block, but not initialized. Meaning: the block of code is aware of the variable, but it cannot be used until it has been declared. Using a let variable before it is declared will result in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until it is declared.