User script which adds a "spoiler" button to the Stack Overflow chat room interface.

soChatSpoilerAdd.user.js

    // ==UserScript==
    // @name        spoiler button adder (SO Chat)
    // @namespace   about:blank
    // @include     http://chat.stackoverflow.com/rooms/*
    // @version     1
    // @grant       none
    // ==/UserScript==
     
    //adds a spoiler to the input textarea.
    //inserts it wherever your cursor is.
    //any highlighted text goes inside the spoiler.
    function add_spoiler(){
        var input = document.getElementById("input");
        var left = input.value.slice(0, input.selectionStart);
        var middle = input.value.slice(input.selectionStart, input.selectionEnd);
        var right = input.value.slice(input.selectionEnd);
     
        if (middle.length == 0){
            middle = "insert spoiler text here";
        }
        middle = "[hover for spoilers](http://www.example.com \"" + middle + "\")";
        input.value = left + middle + right;
        input.setSelectionRange(left.length + middle.length, left.length + middle.length);
        input.focus();
    }
     
    //creates a button that goes next to the input text area.
    //todo: add title text
    function create_button(name, id, func){
        var buttonArea = document.getElementById("chat-buttons");
     
        var button = document.createElement("button");
        button.className = "button";
        button.id = id;
        button.innerHTML = name;
        button.onclick = func;
        buttonArea.appendChild(button);
    }
     
    create_button("spoiler", "spoiler-button", add_spoiler);