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Spreading props in JSX on right hand side of && operator gives incorrect undefined warning

See original GitHub issue

Bug Report

🔎 Search Terms

Spread, Spreading, &&

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about prop spreading.

⏯ Playground Link

💻 Code

import React from "react";

type ComponentOneProps = {
  title: string;

const ComponentOne = ({ title }: ComponentOneProps) => {
  return <h1>{title}</h1>;

type ComponentTwoProps = {
  titleTwo?: string;

const ComponentTwo = ({ titleTwo }: ComponentTwoProps) => {
  const componentOneProps = {
    title: titleTwo,
  return titleTwo && <ComponentOne {...componentOneProps} />;

export default ComponentTwo;

🙁 Actual behavior


Because we checking that titleTwo is truthy here (as it’s the first operand of the logical AND operator) - the title prop of ComponentOne can never actually be set to undefined, but we get a warning saying type Type 'string | undefined' is not assignable to type 'string'.

🙂 Expected behavior

I would expect no error here, as the title prop can never be actually set to a falsey value on ComponentOne there. I think this may be an issue with spreading the props onto ComponentOne here as it works if you set the title prop directly:


Issue Analytics

  • State:open
  • Created 2 years ago
  • Comments:9 (2 by maintainers)

github_iconTop GitHub Comments

MartinJohnscommented, Mar 18, 2022

Here’s a simplified example of what you’re trying to do:

const value = undefined as string | undefined
const obj = { value }
if (value) {
  const expected: string = obj.value

Narrowing the value you stored in an object previously will not narrow the object.

fatcerberuscommented, Sep 23, 2022

@karlhorky See this comment, which in turn links to this comment chain.

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