sample: get multiple elements by id

HTML

<p id="num1">11111</p>
<p id="num2">22222</p>
<p id="num3">33333</p>

JavaScript

//(2) idがnumという文字列で始まる要素（3つ全て）に下線
p = document.querySelectorAll("[id^=num]");

for(var i = 0; i < p.length; i++) {
	p[i].style.textDecoration = "underline";
}