test.js

const cartItem = [
    {
        title: 'Book1',
        exist: false
    }, {
        title: 'Book2',
        exist: true
    }, {
        title: 'Book3',
        exist: true
    }, {
        title: 'Book4',
        exist: false
    },
]

function sortProcusts(carts) {
    carts.sort(function(a, b) {
        if(a.exist === true && b.exist === false){
            return -1
        } else if(a.exist === false && b.exist === true) {
            return 1
        } else {
            return 0
        }
    })
}

sortProcusts(cartItem)
console.log(cartItem)