Hoisting in JavaScript

Hoisting in JavaScript

A simple way to think about hoisting

When your JavaScript runs, the engine first scans your code and registers all the declarations (functions and variables). This makes some things available earlier than you might expect.

Hoisting doesn’t move your code. It just means declarations are known before execution starts. Initial values are not.

What gets hoisted?

• Function declarations are fully hoisted (you can call them before their definition line).
• var variables are hoisted and set to undefined until the assignment runs.
• let and const are hoisted too, but they stay in the Temporal Dead Zone (TDZ) until their declaration line executes. Accessing them early throws an error.

Function declarations (fully available before their line)

var hoisting (declared early, initialized as undefined)

What actually happens under the hood is similar to:

let / const and the Temporal Dead Zone (TDZ)

let and const are hoisted but not initialized. Touching them before their declaration line throws a ReferenceError.

Function expressions and arrow functions

Only the variable name is hoisted, not the function value.

• With var, the variable is undefined until assignment, so calling it early fails at runtime.
• With let/const, accessing it early is a ReferenceError due to TDZ.

A quick mental model

• Think of a classroom before students arrive. The seating chart (declarations) is already set up.
• function declarations: the teacher and the lesson are fully ready—you can call on them immediately.
• var: the seat exists but has no student yet (undefined) until they walk in.
• let/const: the seat is reserved but off-limits (TDZ) until the student arrives at that exact moment.

Practical tips

• Prefer const by default; use let when you need to reassign.
• Avoid var in modern code—its hoisting to undefined and function scope can surprise you.
• Declare variables at the top of their block and initialize them as early as possible.
• When in doubt, move declarations above their first use to avoid TDZ and hoisting confusion.