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typescript

TS18048: 'X' is possibly 'undefined'

TypeScript's noUncheckedIndexedAccess option is enabled and you're using an array element or object value without first confirming it exists. When this option is on, array indexing and object record access returns T | undefined instead of T.

Common causes

noUncheckedIndexedAccess is enabled in tsconfig.json (common in strict projects)
Accessing arr[0] directly when the array might be empty
Accessing a value from a Record<string, T> by key without checking if the key exists
Using an array index calculated at runtime without bounds checking

How to fix it

Check for undefined before using: if (arr[0] !== undefined) { use(arr[0]) }
Use optional chaining: arr[0]?.property
Provide a fallback: const item = arr[0] ?? defaultValue
Destructure with a default: const [first = defaultValue] = arr

Example

const items = ['a', 'b', 'c'];
const first = items[0];
console.log(first.toUpperCase());
// error TS18048: 'first' is possibly 'undefined'.

Array element access when noUncheckedIndexedAccess makes the result T | undefined

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