How to get the value of an element is not set in the style and style sheet style?

Scene: the need to determine whether an element is a fixed location, but its position is not set in the tag style attribute of the element, and how to judge its position is fixed?

Measures: window.getComputedStyle (element, [pseudoElt]) (pseudoElt Alternatively, the pseudo-element indicates the specified node (: before,: after,: first-line,: first-letter, etc.)) to determine the style.