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JavaScriptBeginner#scope
JavaScriptBeginner#scope#es6
Difference between var, let and const?
var is function scoped and hoisted as undefined. let and const are block scoped and stay in the temporal dead zone until initialised. const cannot be reassigned, though object contents can still change.
Example
if (true) { var a = 1; let b = 2; }
console.log(a); // 1
console.log(b); // ReferenceError
const o = { n: 1 };
o.n = 2; // allowed
o = {}; // TypeError