How can I get input field in iframe?

I use the below code to get input field in web page,

var a = document.querySelectorAll('input [ type = "text"]');

However, today I found the code can't work in specific condition, and found:

The input field in the webpage can't be seen in "View Source Page" in Firefox, but if I use Firebug, I can see that an iframe is injected into a div tag, and the iframe contains the input field.(this iframe doesn't exist in "View Source Page")

My question is: how can I get the input field in the iframe?

I am new to JavaScript, so apologizing if it's a dumb question.

Try to remove the quotes from type = "text" and try this:

var a = document.querySelectorAll('input[type=text]')