Predict the output · TypeScript

Type Assertions Don't Convert

Easy Predict the output Seen at Microsoft, Stripe, Atlassian

What does this print?

const input: unknown = "42";
const n = input as number;
console.log(n + 1, typeof n);

```
43 number
```
```
421 string
```
```
43 string
```
```
NaN number
```

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Solution

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The output is:

421 string

as is a compile-time instruction to trust you; no conversion happens. At runtime n is still the string "42", so + 1 concatenates. Validate or convert unknown data explicitly (Number(input), a schema library like Zod) instead of asserting.

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