Execute an instruction after finishing another in Javascript

I'm having problems with the Javascript execution method. I have this piece of code:

let array = [];

list.forEach((item_1) => {
  let variable = 0;
  fetch("data.json")
    .then((resp) => resp.json())
    .then((data) => {
      data.forEach((item_2) => {
        item_2.forEach((item_3) => {
          item_3forEach((item_4) => {
            if (condition) {
              variable += 1;
            }
          });
        });
      });
    });
  array.push({a:variable, b: item_1});
});

I need the variable to be append only when the iterations have finished, however when I run the project, the variable append first (with initial value 0) and then do the iterations. How could I make it append only when the process is finished?

EDIT: Due I wasn't very clear before, I share the piece of code that I ask the question about

for (let c = -100; c < 501; c += 100) {
  fetch(`data/votes_${c + 101}-${c + 200}.json`)
    .then((resp) => resp.json())
    .then((data_vot) => {
      proy_etiq.forEach((eti_selec) => {
        let cont_si_tot = 0;
        data_vot.congresista.forEach((congresista) => {
          congresista.proyecto.forEach((proyecto) => {
            let cont_si = 0;
            eti_selec.proyectos.forEach((proy_selec) => {
              if (proyecto.id === proy_selec.id) {
                cont_si += proyecto.voto.si;
                proy_selec.votos[0] += cont_si;
              }
            });
            cont_si_tot += cont_si;
          });
        });
        flor_si.push({ Tema: eti_selec.nombre, abs: cont_si_tot });
      });
    });
}

The proy_etiq is a list that I alredy have,

Copyright Notice：Content Author:「Juan Pablo Rocha」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/61765029/execute-an-instruction-after-finishing-another-in-javascript