The eval and document.getElementById in JS

JS Eval and document.getElementById can convert strings into corresponding objects or values, but there are some differences between the two, please see the following examples:
If there is a tag with ID AAA on the page, the following a and B get the same result
var a=eval (' aaa ');
var B=document.getelementbyid (' aaa ');
var a=eval (' aaa ');
var B=document.getelementbyid (' aaa ');
If no AAA mark exists on the page, then Eval will have an error and document.getElementById return a null value

Another scenario is when a string is an expression
Alert (eval (' 1+1 ')); Return 2
Alert (document.getElementById (' 1+1 ')); Returns NULL because getElementById will treat ' 1+1 ' as
Tag ID to find
Alert (eval (' aaa.href ')); will return the triple-a HREF attribute
Alert (document.getElementById (' aaa.href ')); Same as above, returns null