index.js

import moveFocusInside, {focusInside, focusIsHidden} from 'focus-lock';

let lastActiveTrap = 0;
let lastActiveFocus = null;


const focusOnBody = () => (
  document && document.activeElement === document.body
);

const isFreeFocus = () => focusOnBody() || focusIsHidden();


const activateTrap = () => {
  let result = false;
  if (lastActiveTrap) {
    const observed = lastActiveTrap;
    if(!isFreeFocus()) {
      if (observed && !focusInside(observed)) {
        result = moveFocusInside(observed, lastActiveFocus);
      }
      lastActiveFocus = document.activeElement;
    }
  }
  return result;
};

const reducePropsToState = (propsList) => {
  return propsList
    .filter(node => node)
    .slice(-1)[0];
};

const handleStateChangeOnClient = (trap) => {
  lastActiveTrap = trap;
  if (trap) {
    activateTrap();
  }
};

let instances = [];

const emitChange = (event) => {
  if (handleStateChangeOnClient(reducePropsToState(instances))) {
    event && event.preventDefault();
    return true;
  }
  return false;
};

const attachHandler = () => {
  document.addEventListener('focusin', emitChange);
};

const detachHandler = () => {
  document.removeEventListener('focusin', emitChange);
};

const focusLock = {
  on(domNode){
    if (instances.length === 0) {
      attachHandler();
    }
    if (instances.indexOf(domNode) < 0) {
      instances.push(domNode);
      emitChange();
    }
  },

  off(domNode){
    instances = instances.filter(node => node !== domNode);
    emitChange();
    if (instances.length === 0) {
      detachHandler();
    }
  }
};

export default focusLock;