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JavaScript - "forEach is not a function" inside fetch().then(); refuses to be converted to an array

so, I have made use of the fetch API to...

get a JSON File off my local server
insert the contents into a element, with appropriate Values, etc. (currently Non-Functional, but also not relevant)

The JSON File: roughly like this: [["hello.css", "world.css", "why.css"], "doesn't.css", "this.css", "work.css"]

The Conditions: if there is a big Group of Strings in an array, inside the array, then turn that into a <optgroup>.

Where it Fails: in the Initialization of the first forEach function.

What I have found out:

The Recieved Text is treated as an Object, obiously making it fail.
It also fails to convert it into an array. Either it is empty or fails differently. Look further below for my attempts.
It is already parsed, apperently, because with JSON.parse it throws an error, complaining about it.

The Code

fetch('./themes.json').then(theResponse => {
    let themesJSON = theResponse.text();
    console.log(themesJSON); /* All "console.log()" functions are for debugging purposes */
    themesJSON.forEach(elm => {
        let the = '';
        if (elm.length > 1) {
            let optgroup = document.createElement('optgroup');
            elm.forEach(elm => {
                let opt = document.createElement('option');
                opt.value = elm;
                opt.innerText = elm;
                optgroup.insertAdjacentElement('beforeend', opt);

                console.log(opt);
            })
            the.insertAdjacentElement("beforeend", optgroup)

            console.log(optgroup);
        } else {
            let opt = document.createElement('option');
            opt.value = elm;
            opt.innerText = elm;
            the.insertAdjacentElement('beforeend', opt);

            console.log(opt);
        }
        document.getElementById('themeSelection').insertAdjacentElement("beforeend", the);
        console.log(the);
    });
})

The Attempts

This Answer Here, returns an empty array, and is not an object anymore, i think
Another Answer under the same post, exact same result.
whatever this is, only tried the first option, as the other one, well, I didn't fully understand it, result? I don't recall anymore.

about 4 years ago · Juan Pablo Isaza

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Accepted Answer · 2022-08-24T00:06:53.484Z

You are using:

let themesJSON = theResponse.text();

which returns a promise that resolves to a string.

What you need is JavaScript representation of your JSON file.

For that the correct syntax would be like this:

fetch('./themes.json')
.then(theResponse => theResponse.json())
.then(actualArray => {
    console.log(actualArray)
})

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