How to put css code in javascript

i want to add a css code in a javascript function, this is the function that I have

function test1() {
    document.getElementById("p1").innerHTML = "test";
}

and i want this css code in it

p1 {
  text-shadow: 0 1px 0 #ccc,
               0 2px 0 #c9c9c9,
               0 3px 0 #bbb,
               0 4px 0 #b9b9b9,
               0 5px 0 #aaa,
               0 6px 1px rgba(0,0,0,.1),
               0 0 5px rgba(0,0,0,.1),
               0 1px 3px rgba(0,0,0,.3),
               0 3px 5px rgba(0,0,0,.2),
               0 5px 10px rgba(0,0,0,.25),
               0 10px 10px rgba(0,0,0,.2),
               0 20px 20px rgba(0,0,0,.15);
}

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Basically, I have a paragraph (p1) and I use javascript because I made a button which onclick="test1()" so when it clicks it it will change the text to the one I put which is "test", but I don't want the whole paragraph to have the shadows, I only want the 'test' to have it, hope I was clear enough!

thank you