这个 JavaScript 的“要求"是什么? [英] What is this JavaScript "require"?
问题描述
我正在尝试让 JavaScript 读取/写入 PostgreSQL 数据库.我在 GitHub 上发现了这个 项目.我能够获得以下示例代码以在 Node 中运行.
I'm trying to get JavaScript to read/write to a PostgreSQL database. I found this project on GitHub. I was able to get the following sample code to run in Node.
var pg = require('pg'); //native libpq bindings = `var pg = require('pg').native`
var conString = "tcp://postgres:1234@localhost/postgres";
var client = new pg.Client(conString);
client.connect();
//queries are queued and executed one after another once the connection becomes available
client.query("CREATE TEMP TABLE beatles(name varchar(10), height integer, birthday timestamptz)");
client.query("INSERT INTO beatles(name, height, birthday) values($1, $2, $3)", ['Ringo', 67, new Date(1945, 11, 2)]);
client.query("INSERT INTO beatles(name, height, birthday) values($1, $2, $3)", ['John', 68, new Date(1944, 10, 13)]);
//queries can be executed either via text/parameter values passed as individual arguments
//or by passing an options object containing text, (optional) parameter values, and (optional) query name
client.query({
name: 'insert beatle',
text: "INSERT INTO beatles(name, height, birthday) values($1, $2, $3)",
values: ['George', 70, new Date(1946, 02, 14)]
});
//subsequent queries with the same name will be executed without re-parsing the query plan by postgres
client.query({
name: 'insert beatle',
values: ['Paul', 63, new Date(1945, 04, 03)]
});
var query = client.query("SELECT * FROM beatles WHERE name = $1", ['John']);
//can stream row results back 1 at a time
query.on('row', function(row) {
console.log(row);
console.log("Beatle name: %s", row.name); //Beatle name: John
console.log("Beatle birth year: %d", row.birthday.getYear()); //dates are returned as javascript dates
console.log("Beatle height: %d' %d"", Math.floor(row.height/12), row.height%12); //integers are returned as javascript ints
});
//fired after last row is emitted
query.on('end', function() {
client.end();
});
接下来我尝试让它在网页上运行，但似乎什么也没发生.我检查了 JavaScript 控制台，它只是说需要未定义".
Next I tried to make it run on a webpage, but nothing seemed to happen. I checked on the JavaScript console and it just says "require not defined".
那么这个要求"是什么?为什么它在 Node 中有效，而在网页中无效?
So what is this "require"? Why does it work in Node but not in a webpage?
此外，在我让它在 Node 中工作之前，我必须执行 npm install pg.那是关于什么的?我查看了目录，没有找到文件 pg.它放在哪里，JavaScript 是如何找到它的?
Also, before I got it to work in Node, I had to do npm install pg. What's that about? I looked in the directory and didn't find a file pg. Where did it put it, and how does JavaScript find it?
推荐答案
那么这个要求"是什么?
So what is this "require?"
require() 不是标准的一部分JavaScript API.但是在 Node.js 中，它是一个具有特殊用途的内置函数:加载模块.
模块是一种将应用程序拆分为单独文件的方法，而不是将所有应用程序放在一个文件中.这个概念也存在于其他在语法和行为上有细微差别的语言中，比如 C 的 include、Python 的 import 等等.
Modules are a way to split an application into separate files instead of having all of your application in one file. This concept is also present in other languages with minor differences in syntax and behavior, like C's include, Python's import, and so on.
Node.js 模块和浏览器 JavaScript 之间的一大区别是如何从另一个脚本的代码访问一个脚本的代码.
One big difference between Node.js modules and browser JavaScript is how one script's code is accessed from another script's code.
在浏览器 JavaScript 中，脚本是通过
