**TypeError: Cannot read property 'name' of undefined**

Most common JavaScript error. Ever.

## Why It Happens

You try to access a property on something that doesn't exist.

```javascript
const user = undefined;
console.log(user.name); // boom
```

## Common Scenarios

**1. API returns nothing**
```javascript
const response = await fetch('/api/user');
const data = await response.json();
console.log(data.user.name); // data.user might be undefined
```

**2. Array element doesn't exist**
```javascript
const users = [{name: 'John'}];
console.log(users[5].name); // users[5] is undefined
```

**3. Object property missing**
```javascript
const config = { theme: 'dark' };
console.log(config.user.name); // config.user is undefined
```

## Fix #1: Check Before Access

```javascript
if (user && user.name) {
  console.log(user.name);
}
```

Ugly but works.

## Fix #2: Optional Chaining (Modern JS)

```javascript
console.log(user?.name);
```

If user is undefined, returns undefined instead of throwing error.

Deep nesting:
```javascript
console.log(user?.address?.street?.name);
```

One undefined anywhere? Returns undefined safely.

## Fix #3: Default Values

```javascript
const name = user?.name || 'Guest';
```

Or with nullish coalescing:
```javascript
const name = user?.name ?? 'Guest';
```

Difference: `??` only defaults on null/undefined, not on 0 or empty string.

## Fix #4: Destructuring with Defaults

```javascript
const { name = 'Guest', age = 0 } = user || {};
```

Even if user is undefined, you get defaults.

## Fix #5: Try-Catch (Last Resort)

```javascript
try {
  console.log(user.name);
} catch (error) {
  console.log('User not found');
}
```

Don't do this unless you have no choice. Slow and ugly.

## Real Example

I was building a dashboard. Users had optional profile pictures:

**Bad code:**
```javascript
<img src={user.profile.picture.url} />
```

New users had no profile object. App crashed.

**Fixed:**
```javascript
<img src={user?.profile?.picture?.url || '/default.png'} />
```

## The Array Problem

```javascript
const users = await fetchUsers();
const firstUser = users[0];
console.log(firstUser.name); // what if users is empty?
```

**Better:**
```javascript
const firstUser = users?.[0];
console.log(firstUser?.name);
```

Note: `users?.[0]` also works with optional chaining!

## Function Calls

```javascript
user.getName(); // if user is undefined, crashes
```

**Fix:**
```javascript
user?.getName?.();
```

Checks if user exists AND if getName is a function.

## The Response Object

Common in fetch calls:
```javascript
const response = await fetch('/api/data');
const data = await response.json();
return data.results[0].title; // 💣
```

Any of these could be undefined. Fix:
```javascript
const title = data?.results?.[0]?.title ?? 'No title';
```

## My Pattern

I validate API responses:
```javascript
function validateUser(data) {
  if (!data || !data.user) {
    throw new Error('Invalid user data');
  }
  return data.user;
}

try {
  const user = validateUser(apiResponse);
  console.log(user.name); // safe now
} catch (error) {
  // handle error properly
}
```

## Browser Support

Optional chaining works in:
- Chrome 80+
- Firefox 74+
- Safari 13.1+
- Node 14+

For older browsers, Babel transpiles it.

## Bottom Line

1. Use optional chaining (`?.`)
2. Provide defaults (`??`)
3. Validate data from APIs
4. Never assume nested objects exist

This error cost me a production outage once. Never again.

Getting 'Cannot read property of undefined'? Here's why it happens and patterns to prevent it in your code.

Cannot Read Property of Undefined - The Most Common JS Error

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Cannot Read Property of Undefined - The Most Common JS Error