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easyTechnical2024

Local storage storage sync events

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01

Understand the problem

Implement the scenario utility: Local storage storage sync events. Discuss edge cases, runtime performance, and alternative paradigms.

js-codingutilityalgorithm
02

Attempt it yourself

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03

Study the solution

💡 How to Think About This Problem

When implementing Local storage storage sync events in a frontend interview, candidate developers should focus on structural correctness, managing execution bindings, and minimizing performance overhead.

1. Deconstructing the Requirements

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Solution ready — 2 min read

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04

Read the code

Example usage
function onStorageChange(key, cb) {
  if (typeof window === 'undefined' || !window.addEventListener) {
    return () => {};
  }
  
  const listener = (e) => {
    // storage event fires on OTHER windows/tabs
    if (e.key === key && e.storageArea === localStorage) {
      cb(e.newValue, e.oldValue);
    }
  };
  
  window.addEventListener('storage', listener);
  
  // Custom hook for same-tab updates (since standard storage listener only fires cross-tab)
  const originalSetItem = localStorage.setItem;
  localStorage.setItem = function(k, v) {
    const oldValue = localStorage.getItem(k);
    originalSetItem.apply(this, arguments);
    if (k === key && oldValue !== v) {
      cb(v, oldValue);
    }
  };
  
  return () => {
    window.removeEventListener('storage', listener);
    localStorage.setItem = originalSetItem; // restore original hook
  };
}

// Example placeholder (requires browser environment)
// const cancel = onStorageChange('user-token', (newV, oldV) => console.log('token updated:', newV));
05

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