Why can I not use getElementById as a predicate to Array#map?

To map an array of stringified numbers to actual Numbers, I can simply pass the Number function:

let arr = ["0", "1", "-2.5"];

console.log(arr.map(Number));

Now I would like to use the same approach with document.getElementById to map a list of id strings to their corresponding DOM nodes:

let arr = ["a", "b"];

console.log(arr.map(document.getElementById));

<div id="a">a <span id="b">b</span></div>

which gives me

"TypeError: 'getElementById' called on an object that does not implement interface Document."

Can someone explain the error?