Is the stored value of the array the original value after being assigned?

  node.js, question
arr[i][0] = arr[i][0].toUpperCase();
 //The original value is' A', and the return value after assignment is' A', but it is still' A' when printed out.

It shouldn’t be what you said. Is there any other implementation details hidden? For example, you used itimmutable.jsLibraries like that?