text-decoration has a default color, usually your browser's (black)

You can trim of the rgb part of the string with .replace and a bit of regex.

This will cut out the rgb part regardless of the rgb(x.x.x) values, as you wanted.

BUT! You wont be able to get initial as a result, because initial replaces to the default none value at text-decoration

style = window.getComputedStyle(document.getElementById('gettextDecorationfrom'));
weight = style.textDecoration;
text = weight.replace(/ ?rgb\(([^;]*)\)/gm, '');
document.getElementById("results").innerText = text;

.myclass {
  text-decoration:underline; 
}

<div id="gettextDecorationfrom" class="myclass">
  Test
</div>
<hr>
Result: <div id="results"></div>

Per the docs, text-decoration is a shorthand property. See: https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration

That means it contains multiple properties. Specifically, it contains text-decoration-line, text-decoration-color, text-decoration-style, and text-decoration-thickness. That's why you see the RGB value, because it includes color in the text-decoration property.

If you just want the text-decoration-line property, then you can use textDecorationLine, but that still will never give you initial as you mentioned in your first example.

That's because getComputedStyle does not return what is in the CSS; it returns the style that actually ends up getting applied the element. That's why it's called computed style. initial is not a computed style, it's a value that instructs the browser to compute the style by using whatever the initial style was, which for text-decoration-line is none, as you can see here: https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration-line#formal_definition

If you want to get that initial, you will have to actually inspect the stylesheet itself. See: https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet

Using a combination of both answers I was able to get what i needed. The answer by Instance Hunter had the JavaScript textDecorationLine i was missing which allowed me to use FHJTDSDG's answer without the regex.

Working example..

updateresult()

function toggleunderline() {
if (weight === "none") {
document.getElementById("gettextDecorationfrom").style.textDecorationLine = "underline"
updateresult()
} else {
document.getElementById("gettextDecorationfrom").style.textDecorationLine = "initial"
updateresult()
}
}

function updateresult(){
style = window.getComputedStyle(document.getElementById('gettextDecorationfrom'));
weight = style.textDecorationLine;
document.getElementById("results").innerText = weight;
}

.myclass {
text-decoration:initial; 
}

<button onclick="toggleunderline()">
Toggle underline
</button>
<div id="gettextDecorationfrom" class="myclass">
Test
</div>
<hr>
Result: <div id="results"></div>