javascript 检查 JSON 对象中是否存在值
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Check if value exists in JSON Object
提问by v1shnu
I am learning NodeJS . I am using a JSON Object to check if a user is present.
我正在学习 NodeJS 。我正在使用 JSON 对象来检查用户是否存在。
This is my JSON (users.json):
这是我的 JSON (users.json)：
{
"users": [{
"fname": "Robert",
"lname": "Downey Jr.",
"password": "ironman"
}, {
"fname": "Chris",
"lname": "Evans",
"password": "cap"
}, {
"fname": "Chris",
"lname": "Hemsworth",
"password": "thor"
}, {
"fname": "Jeremy",
"lname": "Renner",
"password": "hawk"
}]
}
Now I want to pass the fname value of one entry and see if it exists in the JSON Object.
现在我想传递一个条目的 fname 值，看看它是否存在于 JSON 对象中。
This is what I'm doing :
这就是我正在做的：
var file = JSON.parse(fs.readFileSync('users.json', 'utf8'));
for (eachUser in file.users) {
console.log(eachUser.fname);
}
But I do not get the fname values but just undefined. I don't know what I am doing wrong.
但我没有得到 fname 值，而只是undefined. 我不知道我做错了什么。
Also is there a way to find if the value exists without having to iterate over the JSON Object ?
还有一种方法可以在不必遍历 JSON Object 的情况下查找值是否存在？
采纳答案by Kristján
The problem is that eachUsergets set to each index in the array - not the object at that index. If you log just eachUser, you'll see
问题是eachUser设置为数组中的每个索引 - 而不是该索引处的对象。如果你只是登录eachUser，你会看到
0
1
2
3
Instead, pull out the object first, and then gets its fname
相反，先拉出对象，然后再获取它的 fname
for (index in file.users) {
console.log(file.users[index].fname)
}
Here's a demo.
这是一个演示。
If you want to do this lookup without iterating through the array, you can re-shape your data into an object with fnameas the key, so it would look like:
如果您想在不遍历数组的情况下进行此查找，您可以将数据重新整形为一个以fname作为键的对象，因此它看起来像：
{
"Robert": {
"fname": "Robert",
"lname": "Downey Jr.",
"password": "ironman"
},
"Chris": {
"fname": "Chris",
"lname": "Evans",
"password": "cap"
},
"Jeremy": {
"fname": "Jeremy",
"lname": "Renner",
"password": "hawk"
}
}
Of course, then either you won't be able to store any duplicate first names, or your need to make the values arrays.
当然，要么您将无法存储任何重复的名字，要么您需要创建值数组。
回答by NiVeR
if ( typeof file !== 'undefined' && file )
{
//do stuff if file is defined and not null
}
