关于node.js：express.js中的“ app.render”和“ res.render”有什么区别？

What's the difference between “app.render” and “res.render” in express.js?

app.render的文档：

Render a view with a callback responding with the rendered string. This is the app-level variant of res.render(), and otherwise behaves the same way.

res.render的文档：

Render a view with a callback responding with the rendered string. When an error occurs next(err) is invoked internally. When a callback is provided both the possible error and rendered string are passed, and no automated response is performed.

我该如何确定何时使用哪个？

以下是一些区别：

您可以在根级别调用app.render，而只能在路由/中间件内部调用res.render。

app.render始终在回调函数中返回html，而res.render仅在将回调函数指定为第三个参数时才返回。如果在没有第三个参数/回调函数的情况下调用res.render，则呈现的html将以状态码200发送给客户端。

看下面的例子。

app.render

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app.render('index', {title: 'res vs app render'}, function(err, html) {
console.log(html)
});

// logs the following string (from default index.jade)
<!DOCTYPE html><html><head>res vs app render<link rel="stylesheet" href="/stylesheets/style.css"></head><body>res vs app render<p>
Welcome to res vs app render
</p></body></html>

res.render，不带第三个参数

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app.get('/render', function(req, res) {
res.render('index', {title: 'res vs app render'})
})

// also renders index.jade but sends it to the client
// with status 200 and content-type text/html on GET /render

res.render具有第三个参数

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app.get('/render', function(req, res) {
res.render('index', {title: 'res vs app render'}, function(err, html) {
console.log(html);
res.send('done');
})
})

// logs the same as app.render and sends"done" to the client instead
// of the content of index.jade

res.render在内部使用app.render呈现模板文件。

您可以使用render函数创建html电子邮件。根据您应用程序的结构，您可能并不总是具有app对象的权限。

例如在外部路线中：

app.js

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var routes = require('routes');

app.get('/mail', function(req, res) {
// app object is available -> app.render
})

app.get('/sendmail', routes.sendmail);

routes.js

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exports.sendmail = function(req, res) {
// can't use app.render -> therefore res.render
}