How to pass values and make multiple callbacks in nodejs?

Question

I am a beginner to javascript syntax and I'm trying to understand asynchronous callbacks in nodejs. I'm trying to execute this small piece of code which contains 2 basic functions one after the other.

function compute (n1,n2,callback){
    callback(n1,n2);
}
function sum(n1,n2,callback){
    console.log(n1+n2);
    callback(n1,n2);
}
function product(n1,n2){
    console.log(n1*n2);
}



compute(5, 3, function() {
    sum(function (){
        product();
    });
});

I get the following error after I try to run it.

C:\Users\HP\Nodejs\node-example\node-infile-module\practice.js:10
    callback(n1,n2);
    ^

TypeError: callback is not a function
    at sum (C:\Users\HP\Nodejs\node-example\node-infile-module\practice.js:10:5)
    at C:\Users\HP\Nodejs\node-example\node-infile-module\practice.js:19:5
    at compute (C:\Users\HP\Nodejs\node-example\node-infile-module\practice.js:6:5)
    at Object.<anonymous> (C:\Users\HP\Nodejs\node-example\node-infile-module\practice.js:18:1)
    at Module._compile (internal/modules/cjs/loader.js:1156:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
    at Module.load (internal/modules/cjs/loader.js:1000:32)
    at Function.Module._load (internal/modules/cjs/loader.js:899:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)
    at internal/main/run_main_module.js:18:47

As nodejs supports closure property which automatically uses the parameters from parent functions without needing to explicitly mentioning, I dont understand why it is not considering my function as a callback. Also I want to implement error first approach after I get this basic right. I read other stackoverflow answers but they seemed too complex. Please help me

blex · Accepted Answer · 2020-12-22 07:29:28Z

1

You need to provide parameters to the sum function (right now, you are passing a function as n1 and that's it) and the product function:

function compute (n1,n2,callback){
    callback(n1,n2);
}
function sum(n1,n2,callback){
    console.log(n1+n2);
    callback(n1,n2);
}
function product(n1,n2){
    console.log(n1*n2);
}
//                     v   v
compute(5, 3, function(n1, n2) {
    sum(n1, n2, product);
    //  ^   ^      ^
});

answered Dec 22, 2020 at 7:29

blex

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2 Comments

manoj kumar Over a year ago

Why isn't it working if i directly call compute(5,3, sum(...)); instead of enclosing it in a new function with args and then calling it?..

blex Over a year ago

Because when you do compute(5, 3, sum);, you are passing a callback (a function). When you do compute(5, 3, sum(...));, you are calling sum, getting its return value (undefined) and passing it to compute. Enclosing it with an anonymous function makes it possible for compute to call that function and give it parameters

Wang Liang · Accepted Answer · 2020-12-22 07:26:07Z

0

function compute (n1,n2,callback){
    callback(n1,n2);
}
function sum(n1,n2,callback){
    console.log(n1+n2);
    callback(n1,n2);
}
function product(n1,n2){
    console.log(n1*n2);
}

compute(5, 3, function() {
    var arg1 = 5;
    var arg2 = 5;
    // The sum function: type of `third argment` is `function`
    sum(arg1, arg2, function (){
        product();
    });
});

Updated

var arg1 = 5;
var arg2 = 5;
sum(arg1, arg2, function (){

answered Dec 22, 2020 at 7:26

Wang Liang

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4 Comments

blex Over a year ago

You forgot to provide parameters to product();

manoj kumar Over a year ago

But what is it with the closure property?.. I read that theres no need to explicitly pass params to child functions.. doesn't that mean we can ignore passing args?..

blex Over a year ago

If you pass the name of the function, yes, there is no need: compute(5, 3, sum); This will work fine. But in your example, you create a brand new anonymous function which does not have n1 or n2 declared anywhere in its context

manoj kumar Over a year ago

yea i initially tried compute(5,3,sum);. But I want product function to be called as a callback to the sum. Thats where i could'nt figure out how to make multiple callbacks with just passing name of the functions.....

Will · Accepted Answer · 2020-12-22 07:29:34Z

0

You need to pass the arguments through the callbacks.

function compute(n1, n2, callback) {
  callback(n1, n2);
}
function sum(n1, n2, callback) {
  console.log(n1 + n2);
  callback(n1, n2);
}
function product(n1, n2) {
  console.log(n1 * n2);
}

compute(5, 3, function (arg1, arg2) {
  sum(arg1, arg2, function (arg1, arg2) {
    product(arg1, arg2);
  });
});

answered Dec 22, 2020 at 7:29

Will

3,2411 gold badge20 silver badges17 bronze badges

1 Comment

manoj kumar Over a year ago

Why isn't it working if i directly call compute(5,3, sum(...)); instead of enclosing it in a new function with args and then calling it?..

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