你将需要创建一个字典。这是你的函数应该看起来的样子。每次你想要添加一个元素-创建一个新的键-值对。

function GramstoMoles()
{
var elements = {h:1, he:2};

GramsState = document.getElementById("GramsChoice").value;

ElementState = 
document.getElementById("ElementChoice").value;

document.getElementById("resultMolarMass").innerHTML 
= parseInt(GramsState) / elements[ElementState];
}

检查您的代码的这个工作片段！
如果您想将H设置为某个框的初始值，请执行
var ElementState = document.getElementById("ElementChoice").value = H;，然后再调用该函数。

function GramstoMoles (){

var H = 1;
var GramsState = document.getElementById("GramsChoice").value;
var ElementState = document.getElementById("ElementChoice").value;
document.getElementById("resultMolarMass").innerHTML = GramsState / ElementState;

}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="GramsToMolesContainer"><div id="GramMolesText">
<font size=3>Grams To Moles</font></div>
<form>
Grams : <input type="text" id="GramsChoice" /><br>
Element : <input type="text" id="ElementChoice" /><br>
<input type="button" onClick="GramstoMoles()" Value="Convert" />
</form>
<p>Molar Mass : <br>
<span id = "resultMolarMass"></span>
</p>
</div>

当你得到var element和var grams时，

var elements = ['H','He','Li','Be',...];
var atomicmasses = [1,4,6,9,.....];
var molarmass = molarmasses[elements.indexOf(element)];
var mole = grams/molarmass;