how can you find items in an array in react?

What you want is to find a value in an array.

You can use includes

    const array = ["23435","87567", "34536","45234","34532","65365"]

    const aConstant = "23435"

    return (<div>{ array.includes(aConstant) ? 'a' : 'b' }</div>)

Same thing with indexOf

    const array = ["23435","87567", "34536","45234","34532","65365"]

    const aConstant = "23435"

    return (<div>{ array.indexOf(aConstant) !== -1 ? 'a' : 'b' }</div>)

You can also try filter

    const array = ["23435","87567", "34536","45234","34532","65365"]

    const aConstant = "23435"

    return (<div>{ Boolean(array.filter( x => x === aConstant)) ? 'a' : 'b' }</div>)

And even find

    const array = ["23435","87567", "34536","45234","34532","65365"]

    const aConstant = "23435"

    return (<div>{ array.find( x => x === aConstant) ? 'a' : 'b' }</div>)