[Vue] Computed property XXX was assigned to but it has no setter.

项目过程中经常遇到如下问题：

[Vue warn]: Computed property "CurrentStep" was assigned to but it has no setter. 意思是：计算属性 CurrentStep 被赋值了，但此它并未定义 set方法，故出现上面错误提示。

vue官网中关于计算属性的示例是这样的:

var vm = new Vue({data: { a: 1 },computed: {// 仅读取, 计算属性的 getteraDouble: function () {return this.a * 2},// 读取和设置, 计算属性的 getter 和 setteraPlus: {get: function () {return this.a + 1},set: function (v) {this.a = v - 1}}}
})
vm.aPlus   // => 2
vm.aPlus = 3
vm.a       // => 2
vm.aDouble // => 4复制代码

可以看出：aDouble计算属性默认只有get函数,仅支持读取；而aPlus计算属性声明了get 和 set函数，支持读取和设置。故当出现上错误提示时，只需按照aPlus计算属性声明方式修正即可解决问题。

注意：如果你为一个计算属性使用了箭头函数，则 this 不会指向这个组件的实例，不过你仍然可以将其实例作为函数的第一个参数来访问。：

computed: {aDouble: vm => vm.a * 2
}
复制代码

不过我还有个问题：如果computed中依赖的不是data中的属性，而是prop中的属性，我们又应该如何处理呢？

答案：由于vue中prop的属性是父组件传递给子组件值，我们并不建议直接操作prop中的属性。你可以在data中添加一个新的映射属性（作为prop属性的映射），并使用watch达成映射。

附上代码如下：

props: ['step'],
data(){return {stepMap: null}
},
computed:{currentStep:{get(){return this.stepMap},set(v){this.stepMap = v}}
},
watch: {step(v) {this.stepMap = v}
}
复制代码

希望能帮助大家，thanks !!!

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