var vs let/const

var vs let/const

The let/const keyword conceals the variable to its own lexical environment/scope. It provides data privacy because sometimes you want to limit the use of a variable to a scope.

The var keyword “hoists” (or lifts up) the variable definition (its name) up all the way to the global scope. Regardless what block-scope your variable is defined in using var, it always becomes available at the uppermost scope.

Variables defined inside a function-scope using the var keyword are concealed to that function's scope.

For these reasons the let keyword is considered to be a “safer” alternative to the var keyword, even though in a few cases they produce exactly the same behavior. The difference is that let/const always conceals to the scope its defined in.

This is also true of callback functions (which are essentially are function-scope.)

const is just like let, except that the value of a variable declared with const keyword cannot be changed.

During the 'hoisting' cycle of the code execution, when var is hoisted/moved up in the scope, its assigned an 'undefined' value initially.

Thus, if a var variable is used before any value is assigned to it, it will have a value of 'undefined' (which will not cause any error if used in console log or compare expressios or few other cases)

'uninitialized' value for let/const

let/const variables, defined in global or function scope (i.e. a scope for which an execution context is created in the JSRuntime's call-stack, not the scopes created by if/for/while statements) - such let/const variables are also hoisted up in the variable envionment of their respective execution context with a value of 'uninitialized'.

When such 'uninitialized' variable is used before any value is assigned to it, javascript will throw an error - ReferenceError: cannot find value of uninitialized (check the exact error). This area of code where a let/const var cannot be used until assigned a value is called Temporal Dead Zone(TDZ).