How to resolve the issue of the echo (with >>) eating up the line indendations?

Question

Consider the below commands typed into the terminal:

$ ls
my-file.js prefix.yml
$ cat prefix.yml
apiVersion: v1
kind: ConfigMap
metadata:
  name: file1-cm
  namespace: default
data:
  my-file.js: |
$ #kindly note the line feeds in the above output
$ cat my-file.js
let a = require('a');
let b = require('b');
module.exports = function(app, cb) {
    console.log(a + b);
}
$ #kindly note the line feeds and indentation
$ cat prefix.yml > file1-cm.yaml; cat my-file.js | while read LINE; do echo "  ${LINE}" >> file1-cm.yaml; done; echo done;
done
$ cat file1-cm.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: file1-cm
  namespace: default
data:
  my-file.js: |
  let a = require('a');
  let b = require('b');
module.exports = function(app, cb) {
  console.log(a + b);
  cb();
  }

I was expecting a relative indentation in file1-cm.yaml file to be equivalent to the indentation my-file.js. Am I missing something in the apis of echo or read? How come cat my-file.js preserve the indendations whereas echo does not?