In the example below, we have a textarea containing some text. When the user selects something using a mouse, we will check what user has selected. For that, we use selectionStart and selectionEnd properties of textarea, and we can get the selected text using formElement.value.substring().

<textarea id="sample_quote" rows="4"  cols="30">
Please select the text of your choice and you will see the selected text below.
</textarea>
 
<div id="selected_output"></div>
 
<script>
 
var sample_quote = document.getElementById('sample_quote')
var selected_output = document.getElementById('selected_output')
sample_quote.addEventListener('mouseup', function(){
    if (this.selectionStart != this.selectionEnd){ // check the user has selected some text inside field
        var selectedtext = this.value.substring(this.selectionStart,this.selectionEnd)
        selected_output.innerHTML = selectedtext
    }
}, false)
 
</script>

There are some other methods that we can use. One of them is window.getSelection() for retrieving what the user has selected on the page. This method is supported by all modern browsers and IE9+.
For example:

var userSelectedText = window.getSelection().toString()

Here, the text which is selected currently on that page is returned by window.getSelection() and it will returns a Selection object containing that data. To retrieve the actual text from the object we have used toString().