[英]When is the function() block necessary in JavaScript?
I think this is a beginner JavaScript question. 我认为这是一个初学JavaScript问题。 Here's a piece of code (taken from Discover Meteor ) to illustrate:
这是一段代码(取自Discover Meteor )来说明:
Meteor.methods({
// why function is necessary here?
post: function(postAttributes) {
var user = Meteor.user(),
// why is not necessary here?
postWithSameLink = Posts.findOne({
url: postAttributes.url
});
// ensure the user is logged in
if (!user)
throw new Meteor.Error(401, "You need to login to post new stories");
// ensure the post has a title
if (!postAttributes.title)
throw new Meteor.Error(422, 'Please fill in a headline');
// check that there are no previous posts with the same link
if (postAttributes.url && postWithSameLink) {
throw new Meteor.Error(302,
'This link has already been posted', postWithSameLink._id);
}
// pick out the whitelisted keys
// and why not below here?
var post = _.extend(_.pick(postAttributes, 'url', 'title', 'message'), {
userId: user._id,
author: user.username,
submitted: new Date().getTime()
});
var postId = Posts.insert(post);
return postId;
}
});
I believe there is a simple explanation for this. 我相信对此有一个简单的解释。 How do I solve this confusion?
我该如何解决这种混乱?
Actually, that's not a function (as much as I know). 实际上,这不是一个功能(据我所知)。 In JS, if you want to bind events with functions, you have to specify references to those functions.
在JS中,如果要将事件与函数绑定,则必须指定对这些函数的引用。 In spite of referring a function, you can create a
self executing function
itself! 尽管引用了一个函数,您可以自己创建一个
self executing function
! So, you could have done this too: 所以,你也可以这样做:
Meteor.methods({
// why function is necessary here?
post: myFunction
});
function myFunction(postAttributes) {
var user = Meteor.user(),
// why is not necessary here?
postWithSameLink = Posts.findOne({
url: postAttributes.url
});
// ensure the user is logged in
if (!user)
throw new Meteor.Error(401, "You need to login to post new stories");
// ensure the post has a title
if (!postAttributes.title)
throw new Meteor.Error(422, 'Please fill in a headline');
// check that there are no previous posts with the same link
if (postAttributes.url && postWithSameLink) {
throw new Meteor.Error(302,
'This link has already been posted', postWithSameLink._id);
}
// pick out the whitelisted keys
// and why not below here?
var post = _.extend(_.pick(postAttributes, 'url', 'title', 'message'), {
userId: user._id,
author: user.username,
submitted: new Date().getTime()
});
var postId = Posts.insert(post);
return postId;
}
However, in 但是,在
postWithSameLink = Posts.findOne({
url: postAttributes.url
});
you are assigning the result of a function to a variable, and you're NOT binding a function to an event. 您正在将函数的结果赋给变量,并且您没有将函数绑定到事件。 However, you can use a self executing function there too, like this:
但是,您也可以使用自执行功能,如下所示:
postWithSameLink = function()
{
return Posts.findOne({ url: postAttributes.url});
}
I dont understand Meteor
but will try to answer you, the answer is in comments 我不明白
Meteor
但会试着回答你,答案就在评论中
Meteor.methods({
// why function is necessary here? Because this is a function definition
// we are defining a function called post in the object Meteor.methods
// post in Meteor.methods is a function so we are specifying that.
post: function(postAttributes) {
var user = Meteor.user(),
// why is not necessary here? Because this is a function call
// you are probably looking for posts with same link using an
// existing function "Posts.findOne" you dont need to specify
// its a function, its already done somewhere else when
// defining Posts.findOne
postWithSameLink = Posts.findOne({
url: postAttributes.url
});
// ensure the user is logged in
if (!user)
throw new Meteor.Error(401, "You need to login to post new stories");
// ensure the post has a title
if (!postAttributes.title)
throw new Meteor.Error(422, 'Please fill in a headline');
// check that there are no previous posts with the same link
if (postAttributes.url && postWithSameLink) {
throw new Meteor.Error(302,
'This link has already been posted', postWithSameLink._id);
}
// pick out the whitelisted keys
// and why not below here? beacuse this is a case of recursive function calls
// Here you are calling a function called "pick" which is a part of
// the object "_" it probably picks the "Post" with such and such credentials
// and returns the result to the function "extend" which is also part of
// object "_". extend will then probably do some transformation to the
// result and assign the returned result to the variable "post"
// note that var post is different from the function post above, dont confuse em
var post = _.extend(_.pick(postAttributes, 'url', 'title', 'message'), {
userId: user._id,
author: user.username,
submitted: new Date().getTime()
});
var postId = Posts.insert(post);
return postId;
}
});
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