Typescript - string' is not assignable to type 'FC

Question

I'm getting the error below:

Type '(props: PropsWithChildren<{ amount: number; }>) => string' is not assignable to type 'FC<{ amount: number; }>'.
  Type 'string' is not assignable to type 'ReactElement<any, any> | null'.ts(2322)

when using the below typescript function, not understanding the issue here, any help is appreciated, thanks!

Code is below,

const MoneyAmount: React.FC<{amount : number}> = (props) => {
    return (
        new Intl.NumberFormat("en-US", {
            style: "currency",
            currency: "USD", 
            maximumFractionDigits: 4
        }).format(props.amount))
}

export default MoneyAmount  ;

please use triple backticks if you want to embed the piece of code. See how I did it in my answer — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Sep 15, 2021 at 6:16

captain-yossarian from Ukraine · Accepted Answer · 2021-09-15 07:23:30Z

Take a look on FC type:

   type FC<P = {}> = FunctionComponent<P>;

    interface FunctionComponent<P = {}> {
        (props: PropsWithChildren<P>, context?: any): ReactElement<any, any> | null;
        propTypes?: WeakValidationMap<P>;
        contextTypes?: ValidationMap<any>;
        defaultProps?: Partial<P>;
        displayName?: string;
    }

This function returns ReactElement<any, any> | null.

WHich in turn is just a jsx with set of properties.

    interface ReactElement<P = any, T extends string | JSXElementConstructor<any> = string | JSXElementConstructor<any>> {
        type: T;
        props: P;
        key: Key | null;
    }

All you need to do is to wrap your return value into a span:

const MoneyAmount: React.FC<{ amount: number }> = (props) => {
  const text = new Intl.NumberFormat("en-US", {
    style: "currency", currency: "USD", maximumFractionDigits: 4
  })
    .format(props.amount)


  return <span>{text}</span>
}

Let's try to use it without FC:

import React from 'react'

const MoneyAmount = (props: { amount: number }) => {
  return (
    new Intl.NumberFormat("en-US", {
      style: "currency",
      currency: "USD",
      maximumFractionDigits: 4
    }).format(props.amount))
}

// Its return type 'string' is not a valid JSX element.
const x = <MoneyAmount amount={42} />

Hence, string is just invalid JSX

Kevin Ghadyani · Accepted Answer · 2023-02-08 04:21:29Z

4

You can wrap your string in <Fragment>, and it will pass typecheck.

The issue is that React is expecting a ReactElement type, and string isn't one of those. Fragment lets you return strings and arrays to components.

answered Feb 8, 2023 at 4:21

Kevin Ghadyani

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