Snippet to convert html table to json (to be used with google chrome or similiar)

table-to-json.js

function tableToJson(table) {
    var data = [];

    // first row needs to be headers
    var headers = [];
    for (var i=0; i<table.rows[0].cells.length; i++) {
        headers[i] = table.rows[0].cells[i].innerHTML.toLowerCase().replace(/ /gi,'');
    }

    // go through cells
    for (var i=1; i<table.rows.length; i++) {

        var tableRow = table.rows[i];
        var rowData = {};

        for (var j=0; j<tableRow.cells.length; j++) {

            rowData[ headers[j] ] = tableRow.cells[j].innerHTML;

        }

        data.push(rowData);
    }       

    return data;
}

JSON.stringify(tableToJson($0));

Very nice example, but only works on tables that we are expecting to correctly have used a table with th cells.
If we are expecting a table to have a "header row" but it does not use th cells for this we need to update the code.
Using @GavinRay97's original example this enhances it to work with tables where we aren't sure if they will have th cells.
Usage: parseHTMLTableElem(document.getElementById('first-table'), false OR true);
Return: {'headers: [], 'rows':[]}

// Parse HTML table element to JSON array of objects
function parseHTMLTableElem( tableEl, expectingHeaderRow ) {
	var columns = Array.from( tableEl.querySelectorAll( 'th' ) ).map( it => it.textContent );
	var rows = Array.from( tableEl.querySelectorAll( 'tbody > tr' ) );
	// must check for table that has no th cells, but only if we are told to "expectingHeaderRow"
	if ( columns.length == 0 && expectingHeaderRow ) {
		// get columns for a non-th'd table
		columns = Array.from( tableEl.querySelectorAll( 'tbody > tr' )[ 0 ].children ).map( it => it.textContent )
		// must remove first row as it is the header
		rows.shift();
	}
	const returnJson = {
		'headers': columns,
		'rows': rows.map( row => {
			const cells = Array.from( row.querySelectorAll( 'td' ) )
			return columns.reduce( ( obj, col, idx ) => {
				obj[ col ] = cells[ idx ].textContent
				return obj
			}, {} )
		} )
	};
	// if we were expecting a header row with th cells lets see if we got it
	// if we got nothing lets try looking for a regular table row as the header
	if ( !expectingHeaderRow && returnJson.headers.length == 0 && ( returnJson.rows[ 0 ] && Object.keys( returnJson.rows[ 0 ] ).length === 0 ) ) {
		return parseHTMLTableElem( tableEl, true );
	}
	return returnJson;
}

And another variant that supports custom headers:

function tableToJson(table, headers = []) {
  var data = [];

  // first row needs to be headers
  if(!headers.length) {
    for (var i = 0; i < table.rows[0].cells.length; i++) {
      headers[i] = table.rows[0].cells[i].innerHTML.toLowerCase().replace(/ /gi, '');
    }
  }

  // go through cells
  for (var i = 1; i < table.rows.length; i++) {

    var tableRow = table.rows[i];
    var rowData = {};

    for (var j = 0; j < tableRow.cells.length; j++) {

      rowData[headers[j]] = tableRow.cells[j].innerHTML;

    }

    data.push(rowData);
  }

  return data;
}

JSON.stringify(tableToJson(document.getElementsByTagName("table")[0], ["nr", "title", "description"]));

joining the party,

function tableToObject(table) {
    let trs = table.querySelectorAll('tr');
    let headers = Array.from(trs[0].querySelectorAll('th')).map(th => th.textContent.trim());
    let ret = [];
    for (let i = 1; i < trs.length; i++) {
        let obj = {};
        let tds = trs[i].querySelectorAll('td');
        for (let j = 0; j < tds.length; j++) {
            obj[headers[j]] = tds[j].textContent.trim();
        }
        ret.push(obj);
    }
    return ret;
}