JavaScript Function Types and How Hoisting Rules Differ for Each

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JavaScript offers four main ways to define functions: function declarations, function expressions, arrow functions, and IIFEs, each behaving differently during hoisting. Function declarations are fully hoisted, meaning they can be called before they appear in the code. Function expressions and arrow functions stored in variables are not hoisted as callable functions; those using const or let fall into the Temporal Dead Zone, causing a ReferenceError, while var-based ones return undefined and trigger a TypeError. Hoisting refers to JavaScript processing declarations before code execution, but it does not physically move code — only the declaration is made early, not the assigned value. IIFEs are function expressions that execute immediately upon creation and follow the same hoisting limitations as other function expressions.

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