Get the text content selected by the mouse

$(document).on('mouseup',function(){
    console.log(window.getSelection().toString());
});

Supplement: Mark the selected content

$(document).on('mouseup',function(e){  
        let a = document.createElement("span");
        a.setAttribute("class","wrap");
        window.getSelection().getRangeAt(0).surroundContents(a);
    });

<style>
    span.wrap{background:goldenrod;}
</style>