b.includes("Verhuurd"); document.querySelector(".nf-form-cont").classList.add("displaynone");

You are just adding the class in any case here, you did not make this dependent on any condition.

This should be something like

if( b.includes("Verhuurd") ) { document.querySelector(".nf-form-cont").classList.add("displaynone"); }

Your includes() is probably matching “huur” and therefore including divs that contain “Te huur” as well as the ones you’re looking for. Consider making it more specific with another condition to exclude divs that you don’t want.

window.addEventListener("load", () => {
    let b = document.getElementsByClassName('aw-property-data-item')[0].innerHTML;

    if (b.includes("Verhuurd") && !b.includes("Te")) {
        document.querySelector(".nf-form-cont").classList.add("displaynone");
    }
});

How can I add a class to a div when another div on the same page contains a specific word?

My html is:

window.addEventListener("load", () => { let b = document.getElementsByClassName('aw-property-data-item')[0].innerHTML; b.includes("Verhuurd"); document.querySelector(".nf-form-cont").classList.add("displaynone"); });

<div class="aw-property-data-item">
<p><strong>Status:</strong>  Verhuurd</p>
</div>

<div id="nf-form-7-cont" class="nf-form-cont">
form content goes here
</div>

This works, the class displaynone is added.

On objects that contain “Te Huur” rather than “Verhuurd” the class displaynone is also added. This should not be the case, but I can’t figure out why this is happening. Any advice is very much appreciated!