contentscript.js

//https://gist.github.com/srsudar/e9a41228f06f32f272a2

'use strict';

/**
 * Insert the text at the cursor into the active element. Note that we're
 * intentionally not appending or simply replacing the value of the editable
 * field, as we want to allow normal pasting conventions. If you paste at the
 * beginning, you shouldn't see all your text be replaced.
 * Taken from:
 * http://stackoverflow.com/questions/7404366/how-do-i-insert-some-text-where-the-cursor-is
 */
function insertTextAtCursor(text) {
    var el = document.activeElement;
    var val = el.value;
    var endIndex;
    var range;
    var doc = el.ownerDocument;
    if (typeof el.selectionStart === 'number' &&
        typeof el.selectionEnd === 'number') {
        endIndex = el.selectionEnd;
        el.value = val.slice(0, endIndex) + text + val.slice(endIndex);
        el.selectionStart = el.selectionEnd = endIndex + text.length;
    } else if (doc.selection !== 'undefined' && doc.selection.createRange) {
        el.focus();
        range = doc.selection.createRange();
        range.collapse(false);
        range.text = text;
        range.select();
    }
}

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    if (request.data) {
        insertTextAtCursor(request.data);
    }
});