如何合并对象数组中的重复项并对特定属性求和？ [复制]

我有这个对象数组：

var arr = [
    {
        name: 'John',
        contributions: 2
    },
    {
        name: 'Mary',
        contributions: 4
    },
    {
        name: 'John',
        contributions: 1
    },
    {
        name: 'Mary',
        contributions: 1
    }
];

...我想合并重复项，但将它们的贡献相加。结果如下：

var arr = [
    {
        name: 'John',
        contributions: 3
    },
    {
        name: 'Mary',
        contributions: 5
    }
];

我怎样才能用 JavaScript 实现这一点？

您可以使用哈希表并生成一个包含所需总和的新数组。

var arr = [{ name: 'John', contributions: 2 }, { name: 'Mary', contributions: 4 }, { name: 'John', contributions: 1 }, { name: 'Mary', contributions: 1 }],
    result = [];

arr.forEach(function (a) {
    if (!this[a.name]) {
        this[a.name] = { name: a.name, contributions: 0 };
        result.push(this[a.name]);
    }
    this[a.name].contributions += a.contributions;
}, Object.create(null));

console.log(result);