javascript - How do I change variable in a function from another file? - Stack Overflow

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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?

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?

• javascript
• node.js

Share Improve this question Follow edited Jun 24, 2017 at 4:42 user663031 asked Jun 24, 2017 at 4:16 Alan HuangAlan Huang 5111 silver badge77 bronze badges

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 6

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.

Global

You can make a global. This is not remended 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;

Pass and Return

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;
};

Put value in object and pass reference to object

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./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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