SOURCE

/*
*数组切割与合并
*/
let b_content = ["A2", "B2"];
let content = ["A", "B", "C", "D", "E"];
let key = content.findIndex(function (item) {
        return item == 'B';
});
let old_content = content.slice(key); 

let new_content = b_content.concat(old_content); 
console.log(new_content)

console.log(key)