在JavaScript中有条件地在Array中添加元素 [英] Add elements inside Array conditionally in JavaScript
问题描述
当我尝试有条件地使用spread运算符合并两个对象时,它在条件为 true
或 false
:
When I try to merge two objects using the spread operator conditionally, it works when the condition is true
or false
:
let condition = false;
let obj1 = { key1: 'value1'}
let obj2 = {
key2: 'value2',
...(condition && obj1),
};
// obj2 = {key2: 'value2'};
当我尝试对数组使用相同的逻辑时,它仅在条件为<$ c时有效$ c> true :
When I try to use the same logic with Arrays, it only works when the condition is true
:
let condition = true;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition && arr1)];
// arr2 = ['value2', 'value1']
如果条件是 false
则抛出错误:
If the condition is false
an error is thrown:
let condition = false;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition && arr1)];
// Error
为什么 Array $ c之间的行为不同$ c>和
对象
?
推荐答案
当你传播到 array ,在对象上调用 Symbol.iterator
方法。 &&
评估为第一个假值(或最后一个真值,如果所有都是真的),所以
When you spread into an array, you call the Symbol.iterator
method on the object. &&
evaluates to the first falsey value (or the last truthy value, if all are truthy), so
let arr2 = ['value2', ...(condition && arr)];
结果
let arr2 = ['value2', ...(false)];
但 false
没有 Symbol.iterator
方法。
您可以使用条件运算符,如果条件为false,则传播空数组:
You could use the conditional operator instead, and spread an empty array if the condition is false:
let condition = false;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition ? arr1 : [])];
console.log(arr2);
(这是因为空数组 具有 Symbol.iterator
方法)
(This works because the empty array does have the Symbol.iterator
method)
对象传播完全不同:它将自己的可枚举属性从提供的对象复制到新对象上。 false
没有任何自己的可枚举属性,因此不会复制任何内容。
Object spread is completely different: it copies own enumerable properties from a provided object onto a new object. false
does not have any own enumerable properties, so nothing gets copied.
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