<body>
    <h1>Copy Excel Paste Markdown</h1>
    <textarea id="editor"></textarea>
    <script src="script.js"></script>
</body>

</html>
textarea {
    width: 800px;
    height: 500px;
    font-family: "Lucida Console";
}

var editor = document.getElementById("editor");

function columnWidth(rows, columnIndex) {
    return Math.max.apply(
        null,
        rows.map(function (row) {
            return ("" + row[columnIndex]).length;
        })
    );
}

function looksLikeTable(data) {
    return true;
}

editor.addEventListener("paste", function (event) {
    var clipboard = event.clipboardData;
    var data = clipboard.getData("text/plain").trim();

    if (looksLikeTable(data)) {
        event.preventDefault();
    } else {
        return;
    }

    var rows = data.split(/[\u0085\u2028\u2029]|\r\n?/g).map(function (row) {
        row = row.replace("\n", " ");
        return row.split("\t");
    });

    var colAlignments = [];

    var columnWidths = rows[0].map(function (column, columnIndex) {
        var alignment = "l";
        var re = /^(\^[lcr])/i;
        var m = column.match(re);
        if (m) {
            var align = m[1][1].toLowerCase();
            if (align === "c") {
                alignment = "c";
            } else if (align === "r") {
                alignment = "r";
            }
        }
        colAlignments.push(alignment);
        column = column.replace(re, "");
        rows[0][columnIndex] = column;
        return columnWidth(rows, columnIndex);
    });
    var markdownRows = rows.map(function (row, rowIndex) {
        // | Name         | Title | Email Address  |
        // |--------------|-------|----------------|
        // | Jane Atler   | CEO   | jane@acme.com  |
        // | John Doherty | CTO   | john@acme.com  |
        // | Sally Smith  | CFO   | sally@acme.com |
        return (
            "| " +
            row
                .map(function (column, index) {
                    return (
                        column +
                        Array(columnWidths[index] - column.length + 1).join(" ")
                    );
                })
                .join(" | ") +
            " |"
        );
        row.map;
    });
    markdownRows.splice(
        1,
        0,
        "|" +
            columnWidths
                .map(function (width, index) {
                    var prefix = "";
                    var postfix = "";
                    var adjust = 0;
                    var alignment = colAlignments[index];
                    if (alignment === "r") {
                        postfix = ":";
                        adjust = 1;
                    } else if (alignment == "c") {
                        prefix = ":";
                        postfix = ":";
                        adjust = 2;
                    }
                    return (
                        prefix +
                        Array(columnWidths[index] + 3 - adjust).join("-") +
                        postfix
                    );
                })
                .join("|") +
            "|"
    );

    // https://www.w3.org/TR/clipboard-apis/#the-paste-action
    // When pasting, the drag data store mode flag is read-only, hence calling
    // setData() from a paste event handler will not modify the data that is
    // inserted, and not modify the data on the clipboard.

    event.target.value = markdownRows.join("\n");
    return false;
});
View Compiled

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.