I really have hard time to understand react. I try to find headings in my blocks and put them in a constant. So far so good. I can confirm this works when I do a console.info(headings) after I found them in the blocks.

Now I try to save this array as an attribute. It does not matter if I define one in registerBlockType first or if I just try to set them with

props.setAttributes( { headings: headings } );

If I do so, I get this error:

Maximum update depth exceeded. This can happen when a component repeatedly calls setState inside componentWillUpdate or componentDidUpdate. React limits the number of nested updates to prevent infinite loops.

So I think this is some kind of recursion. But why does this happen? If I remove props.setAttributes it works but of course I don't have any attribute I get set my heading from.

registerBlockType( 'simpletoc/toc', {
    title: __( 'SimpleTOC', 'simpletoc' ),
    icon: listulicon,
    category: 'layout',
    attributes: {
        headings: {
            type: 'array'
        },
    },
    edit: function ( props ) {
        const headings = find_headings( props );
        console.info(headings)
        props.setAttributes( { headings: headings } );

        return <Toc {...props} />;
    },
    save: function ( props ) {
        return <Toc  {...props} />;
    },
} );

function Toc( props ) {
    return <p className={ props.className }>
            <h2 class="simpletoc-title">{ __( 'Table of Contents', 'simpletoc' ) }</h2>
            <ul class="simpletoc">
                { props.headings }
            </ul>
            </p>;
}

function find_headings ( props ){

    const data = wp.data.select( 'core/block-editor' );
    const blocks = data.getBlocks();
    const headings = blocks.map( ( item, index ) => {
        if ( item.name === 'core/heading' ) {
            var slug = '';
            var hashslug = '';
            var blockId = ( item.clientId );

            slug = item.attributes.content;
            //wp.data.dispatch( 'core/editor' ).updateBlockAttributes( blockId, { anchor: slug } );
            hashslug = '#' + slug;

            return <li><a href={hashslug}>{item.attributes.content}</a></li>;
        }
    } );

    return {headings};
}