node.js - Questions about eval in nodejs

Question

I currently want to share an instance variable. For example, to connect to a database, after it is instantiated in main.js, other modules that want to use the database must instantiate it again.

For example main.js

let redisApi;
redisApi = new RedisApi();

user.js

console.log(redisApi);

At this time, an error message will be reported indicating that the redisApi variable is undefined!
But after I switched to using eval to initialize the variable, it was different

main.js

eval (`let redisApi;`);
redisApi = new RedisApi();

At this time, other modules can share the redisApi variable.
Why can eval do this? Can anyone explain it?

Answer 1

Here you need to understand how modules in Node.js are loaded.
Similar to browsers, there is a global object in the execution environment of Node.js, which is similar to the DOM window object.
Similarly,
Look at the code explanation directly:

//module1.js
console.log(global.testStr)//undefined
testStr = '123';//！！关键点！！，这等于是给全局对象设置了一个名为testStr属性。
console.log(global.testStr)//123
var test = function () {
    console.log(testStr);
}

exports.test = test;

//index.js
require('./module1.js')

console.log(testStr)
//输出:123

So you can understand it by comparing it with your code. What actually takes effect is redisApi = new RedisApi();, eval (`let redisApi;`);The variable declared is in another independent Within the scope, it is actually inaccessible.

Answer 2

Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Program. The eval code for a particular invocation of eval is the global code portion of that Program.

This is a quote from the original text of es6. This is how the specification is set, so you have the result; however, it is generally not recommended to play this way;