What is the difference between var, let, and const?

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All three keywords declare variables, but they differ along three axes: scope , hoisting behaviour , and reassignability . Modern JavaScript code should use const by default, let when reassignment is needed, and avoid var entirely. Scope var — function-scoped. A var declared inside an if or for block leaks out to the enclosing function. let and const — block-scoped. They live only inside the nearest { … } . function demo() { if (true) { var a = 1; let b = 2; } console.log(a); // 1 console.log(b); // ReferenceError } Hoisting and the temporal dead zone var declarations are hoisted and initialised to undefined , so reading them before the declaration is silently valid. let and const are hoisted too, but they stay in the temporal dead zone until the declaration executes. Accessing them early throws a ReferenceError . console.log(x); // undefined var x = 1; console.log(y); // ReferenceError: Cannot access "y" before initialization let y = 1; Reassignment and redeclaration var — can be reassigned and redeclared in the same scope without error. let — can be reassigned but not redeclared in the same scope. const — cannot be reassigned and cannot be redeclared. Note this binds the reference , not the value: a const object's properties are still mutable. const user = { name: "Ada" }; user.name = "Grace"; // allowed — mutating the object user = {}; // TypeError — rebinding the variable Global object attachment A top-level var becomes a property of the global object ( window in browsers). Top-level let and const do not. This matters for avoiding accidental global pollution. Classic loop gotcha Because var is function-scoped, every iteration of a loop shares the same binding. let creates a fresh binding per iteration, which is why callbacks in loops behave intuitively only with let . for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)); // logs: 3, 3, 3 for (let i = 0; i < 3; i++) setTimeout(() => console.log(i)); // logs: 0, 1, 2 The for loop with setTimeout is the #1 var vs let interview question. Know three differences: scope (function vs block), hoisting (undefined vs TDZ), and redeclaration. const does not make objects immutable — only the binding.