You can use the File and FileReader objects to read local files.

You can use an input element with type=file to allow the user to select a file.

<input id=myFile type=file/>

<textarea id=myTextArea rows=4 columns=20></textArea>

After the user has selected a file, you can get the File object from the input element. For example...

var file = document.getElementById(myFile).files[0];

You can then use a FileReader object to read the file into the text area. For example...

var reader = new FileReader();

reader.onload = function (e) {

    var textArea = document.getElementById(myTextArea);

    textArea.value = e.target.result;

};

reader.readAsText(file);