example.js

const leftText = "Hello"

// using props.chilren
const Button = (props) => (
  <div>
   {props.children}
  </div>
)

<Button>
  {leftText}
</Button> 

// OR, but not a good pattern

<Button children={leftText} />

// using props.leftText
const OtherButton = (props) => (
  <div>
   {props.buttonText}
  </div>
)

<OtherButton buttonText={leftText} />