Вариант 1
let centeredDiv = document.querySelector('#centered-div');
let parentDiv = document.querySelector('#parent-div');
let centeredDivWidth = centeredDiv.offsetWidth;
let centeredDivHeight = centeredDiv.offsetHeight;
let parentDivWidth = parentDiv.offsetWidth;
let parentDivHeight = parentDiv.offsetHeight;
if ((centeredDivWidth + centeredDiv.offsetLeft) == (parentDivWidth / 2)
&& (centeredDivHeight + centeredDiv.offsetTop) == (parentDivHeight / 2)) {
console.log('Div центрирован');
} else {
console.log('Div не центрирован');
}
Вариант 2
let centeredDiv = document.querySelector('#centered-div');
let parentDiv = document.querySelector('#parent-div');
let centeredDivWidth = centeredDiv.offsetWidth;
let centeredDivHeight = centeredDiv.offsetHeight;
let parentDivWidth = parentDiv.offsetWidth;
let parentDivHeight = parentDiv.offsetHeight;
if ((parentDivWidth / 2) - (centeredDivWidth / 2) == centeredDiv.offsetLeft
&& (parentDivHeight / 2) - (centeredDivHeight / 2) == centeredDiv.offsetTop) {
console.log('Div центрирован');
} else {
console.log('Div не центрирован');
}