I have some code like this:
//app.js
var r=require("./second.js");
var a=1;
r.second(a);
console.log(a);
And my second.js looks like this:
//second.js
module.exports.change=function(a){
a=2;
}
Currently console.log(a) only show 1, I did a bit of research saying I can't pass it as a variable because variable is passed as value. I thought about using global.a. However, it doesn't work too because a is stored as a module variable, not a global variable, and I don't want to set it as a global variable.
How do I solve it?
The main answer is that you can't do it exactly as you asked. A variable defined as you have a
in app.js is private to the scope of a
and only other code within app.js can modify it.
There are a number of other ways you can structure things such that module B can modify something in module A or cause something to be modified in module A.
You can make a
global. This is not recommended for a variety of reasons. But you could make it a property of the global object by changing your declaration of a
to:
global.a = 1;
Then, in module B, you can directly set:
global.a = 2;
You can pass a
into a function in module B and have that function return a new value that you assign back into a
.
const b = require('moduleB');
let a = 1;
a = b.someFunction(a);
console.log(a); // 2
Then, inside of moduleB, you can modify propertys on a
directly:
// inside of moduleB
module.exports.someFunction = function(a) {
return ++a;
};
Rather than storing the value in a simple variable, you can store it as a property of an object an you can then pass a reference to that object. Objects in Javascript are passed by ptr so some other function you pass the object to can modify its properties directly.
const b = require('moduleB');
let a = {someProperty: 1};
b.someFunction(a);
console.log(a.someProperty); // 2
Then, inside of moduleB, you can modify propertys on a
directly:
// inside of moduleB
module.exports.someFunction = function(obj) {
obj.someProperty = 2;
};
Initially, there isn't any function that is called second
. Your module exposes a function called change
. Then when you pass a value to function this value is copied and whatever change you do affects only the copy. So passing the value of a
, the change affects only the copy of this value and not the original one. What could you do is to pass an object. This time a copy of the reference to the object is passed. So any change you make is reflected back to the original object (it's like pointers in C and C++).
In terms of code, the changes you should make are the following:
// You could make the method more dynamical passing the value you want a get.
module.exports.change = function(obj, value){
obj.a = value;
}
Last you should call it as:
var r=require("./second.js");
var obj = { a: 1};
r.change(obj,2);
console.log(obj.a);
First way is return in your change
function in second.js
.
app.js:
var r=require("./second.js");
var a=1;
a = r.change(a);
console.log(a);
second.js:
module.exports.change=function(a){
a=2;
return a;
}
Second way is use object as parameter to pass it by reference:
app.js:
var r=require("./second.js");
var obj={a:1};
r.change(obj);
console.log(obj.a);
second.js:
module.exports.change=function(obj){
obj.a=2;
}
You can use localStorage or sessionStorage for setting and getting your variable. for node js you can use https://www.npmjs.com/package/localStorage package
//You can set using localStorage.setItem('key','value')
var r = require("./second.js");
var a = 1;
localStorage.setItem('a', 1) /*Window api for setting variable locally
localStorage.setItem('key','value')*/
r.second(a);
console.log(a);
You can get using localStorage.getItem('key')
//second.js
module.exports.change=function(a){
a=localStorage.getItem('a')
}
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