Javascript 阵列拼接正在创建空白阵列
我有以下代码Javascript 阵列拼接正在创建空白阵列,javascript,arrays,reactjs,Javascript,Arrays,Reactjs,我有以下代码 constructor(props) { super(props); this.state = {registrations: [{firstName: "foo", lastName: "bar", activity: "Science Lab", restrictions: "a,b,c"}]} } addRegistration(registration) { console.log('came i
constructor(props) {
super(props);
this.state = {registrations: [{firstName: "foo", lastName: "bar", activity: "Science Lab", restrictions: "a,b,c"}]}
}
addRegistration(registration) {
console.log('came inside event handler to add registration')
console.log(registration)
console.log('existing state')
console.dir(this.state.registrations)
var newState = this.state.registrations.splice() // copy
console.dir(newState)
newState.push(registration)
console.log(newState)
this.setState({registrations: newState})
}
当我运行代码并调用addRegistration事件处理程序时,我看到以下输出
[Log] {firstName: "Foo", lastName: "Bar", activity: "Swimming", restrictions: "a,b"} (Registration.html, line 404)
[Log] existing state (Registration.html, line 405)
[Log] [Object] (1) (Registration.html, line 406)
[Log] [] (0) (Registration.html, line 408)
[Log] [Object] (1) (Registration.html, line 410)
所以我得到一个有效的注册对象作为输入参数。在日志的第3个条目中,我们看到现有状态有一个大小为1的数组。(这是构造函数中的foo对象
接下来,我们看到对this.state.registrations.splice()的调用创建了一个空数组。然后,作为参数接收的新注册对象被推送到创建的空数组中,从而创建另一个大小为1的数组
我不明白为什么splice()创建了一个空数组。它应该返回一个现有状态的副本,然后推送应该导致数组大小为2
我遗漏了什么?返回已删除元素的数组,而方法将数组的一部分的浅层副本返回到从begin
到end
(不包括end)中选择的新数组对象中。您可以使用扩展运算符语法或类似切片的方法创建数组的副本
var newState = this.state.registrations.slice()
或
使用.slice()
复制而不是.splice()
啊……这很简单,谢谢!
var newState = [...this.state.registrations]