Hoisting is JavaScript's behavior of moving declarations to the top of their scope during the compile phase, before any code runs. But how a thing is hoisted differs by declaration type.
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Hoisting is JavaScript's behavior of moving declarations to the top of their scope during the compile phase, before any code runs. But how a thing is hoisted differs by declaration type.
.(a);
a = ;
();
() { .(); }
.(b);
b = ;
var: declaration hoisted and initialized to undefined. So reading it early gives undefined, not an error.function declarations: fully hoisted — you can call them before their definition line.let / const: hoisted but not initialized. They sit in the Temporal Dead Zone (TDZ) from the start of the block until their declaration line; accessing them in the TDZ throws a ReferenceError.const f = () => {}) are not callable early — only the const/var binding is hoisted, not the function.Hoisting explains "why is this undefined?" and "why does calling this function before its definition work?" The TDZ for let/const is actually a feature — it turns silent undefined bugs into loud errors, encouraging you to declare before use.
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