Question

Following javascript code throws error

Uncaught TypeError: Object #<HTMLSpanElement> has no method 'getElementById'

If this is not allowed how do I search for the existence of particular element inside other element?

var googlePlusPosts = document.getElementsByClassName("mo fj");

for (var i = 0; i <  googlePlusPosts .length; ++i) {
        decryptElements = googlePlusPosts[i].getElementById('decrypt-button');
.
.
.

Answer 1

IDs must be unique; thus it doesn’t really make sense to look inside a particular node for a particular ID; either the ID exists, or it doesn’t.

You can use element.getElementsByClassName to get child elements of a particular class, or element.getElementsByTagName to get them by their tag name.

EDIT: Ignore the below; just saved for posterity.

Check if the element has the getElementById method ahead of time.

if (foo.getElementById) {
  foo.getElementById("bar");
}