For most languages, the inner function body when first defined is parsed/compiled once, with the unbound free variables collected in a list. At the next calling

For most languages, the inner function body when first defined is parsed/compiled once, with the unbound free variables collected in a list. At the next calling of the function definition, the compiled function body is reused and only the free variables are bounded to the variables in the outer environment (closure). It's quite efficient. Depending on the implementation, it could just be a two-element struct pointing to the compiled function body and to the outer environment. Or it could be a small call thunk embedding the setup of the variable bindings and the jump to the address of the function body.