Cannot read properties of undefinedCopy Anchor Link

ErrorCopy Anchor Link

TypeError: Cannot read properties of undefined (reading 'foo')

Root CauseCopy Anchor Link

You are trying to access a property on a value that is undefined. This happens when:

• A variable was never assigned
• An async operation hasn't resolved yet
• An object doesn't have the expected shape
• An array index is out of bounds

ExampleCopy Anchor Link

const user = undefined;
console.log(user.name); // ❌ TypeError: Cannot read properties of undefined (reading 'name')

FixCopy Anchor Link

Option 1: Optional chaining

console.log(user?.name); // ✅ Returns undefined safely

Option 2: Guard clause

if (user) {
  console.log(user.name);
}

Option 3: Default value

const { name = "Guest" } = user ?? {};

PreventionCopy Anchor Link

• Always initialize variables with a default value
• Use TypeScript — it catches these at compile time
• Use optional chaining (?.) when accessing nested properties from API responses