JavaScript Memory Allocation Phase Explained

JavaScript Memory Allocation Phase Explained

The memory allocation phase is the first half of creating an execution context. It runs before any line of code executes.

What Happens

When the engine enters a new execution context, it scans the code top-to-bottom:

• Variables declared with var: memory is allocated, value is set to undefined.
• Variables declared with let and const: memory is allocated, but they are placed in the Temporal Dead Zone. Accessing them throws a ReferenceError.
• Function declarations: memory is allocated and the entire function body is stored. You can call them before the line they are declared on.
• Function expressions: treated as variables. With var, they get undefined. With let/const, TDZ applies.

Why This Causes Hoisting

Because memory is allocated before execution, you can use variables and functions before their declaration line. That behavior is called hoisting.

console.log(x); // undefined
foo(); // "hello"
var x = 1;
function foo() { console.log("hello"); }

x is undefined because only the declaration was hoisted, not the assignment. foo works because the whole function was hoisted.

Function vs Variable Hoisting

Function declarations are hoisted with their full body. Variables declared with var are hoisted only with undefined. This is why calling a function expression before assignment fails.

The Takeaway

The memory allocation phase scans the code, reserves memory for all declarations, assigns undefined to var variables, places let/const in the TDZ, and stores function declarations in full. This is the root cause of hoisting.