as per documentation,
var elem = document.querySelector('.grid')
Masonry(elem, {
itemSelector: '.grid-item',
columnWidth: '.grid-sizer',
percentPosition: true
})
Should be:
var elem = document.querySelector('.grid')
var msnry = new Masonry( elem, {
itemSelector: '.grid-item',
columnWidth: '.grid-sizer',
percentPosition: true
});