import * as update from 'immutability-helper';

const state1 = {root: [{a: 1}, {b: 2}]};
// 把{b: 2} 修改成 {b: 100}
const state2 = update(state1, {root: {[1]: {b: {$set: 100}}}});
console.log(state2);

输出：

{ root: [ { a: 1 }, { b: 100 } ] }