How to parse string | number in Typescript?

How to parse string | number in Typescript?

Problem Description:

I need add 40 to a string | number type, and return a string | number. How is it possible in Typescript?

I thought parse it to number, then increase it and return it. But how can I parse a variable which is not sure is string, but it can be number? 🙂

tried this:

parseInt(x) – 44

but raised an

Argument of type 'string | number' is not assignable to parameter of type 'string'.
  Type 'number' is not assignable to type 'string'.ts(2345)

Solution – 1

instead of parseInt use Number something like


Solution – 2

By checking the type:

if (typeof x === "string") {
    x = Number(x); // Or `parseInt` or whatever
const result = x - 44;

After the if, TypeScript will know from flow analysis that x has been narrowed from string | number to number.

Playground example

For the parsing, see my answer here for your various options. parseInt stops at the first non-numeric character and returns whatever number it has at that point (rather than returning NaN), which may or may not be your desired behavior.

Solution – 3

To add another option, you could simply make sure that what’s passed to parseInt is a string using a template string:

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