Asynchronous execution inside for loops (Golang and Nodejs)

The following code is written to highlight the challenge of asynchronous execution inside for loops using code snippets in Golang and Nodejs

In Golang

    package main

    import (
        "fmt"
        "runtime"
    )

    func main() {

        runtime.GOMAXPROCS(1)

        for i := 1; i < 3; i++ {
            for j := 1; j < 3; j++ {
                go func() {
                    fmt.Printf("%d + %d = %d\n", i, j, i+j)
                }()
            }
        }

        fmt.Scanln()
    }

Output

(Note that the program is being executed in a single thread because of this line - runtime.GOMAXPROCS(1)). The output will be different if this line is removed as the code (goroutines) can be executed over multiple threads across cores simultaneously

3 + 3 = 6
3 + 3 = 6
3 + 3 = 6
3 + 3 = 6

Understanding code execution in Golang

• When the go code is executed, the main goroutine completes execution before handing over control to other goroutines
• Consequently the two nested for loops complete all their iterations creating a total of 4 child goroutines. Thereafter the execution for the main goroutine pauses at fmt.Scanln() which waits for a keystroke. Execution at this point is transferred to the child goroutines
• When the execution of the child goroutines starts, it refers to the variables - i and j as stored on the main goroutine’s execution stack.
• And after all the iterations the value of i and j is set at 3 where the execution of both the loops terminated (since it did not satisfy the condition i < 3)
• Consequently when the child goroutines are executed they all print 3 + 3 = 6 on the console

To get this program to output all of the combinations of i and j (order not considered)

1 + 1 = 2
1 + 2 = 3
2 + 1 = 3
2 + 2 = 4

We can pass the variables i and j to the respective functions as arguments when they are invoked as follows

go func(i int, j int) {
	fmt.Printf("%d + %d = %d\n", i, j, i+j)
}(i, j)

Output after the above modification

1 + 1 = 2
2 + 2 = 4
2 + 1 = 3
1 + 2 = 3

Understanding code execution after modification

• In the second case, the variables i and j are no longer referred on the main goroutine’s execution stack.
• Instead these variables have been passed to the functions as arguments and thus are referred on the execution stack of the child goroutines

A similar behavior is observed when executing asynchronous tasks in Nodejs.

In Nodejs

for(var i=1; i<3; i++) {
    for(var j=1; j<3; j++) {
        setTimeout(() => {
            console.log(i+" + "+j+" = "+(i+j));
        }, 0)
    }
}

Output

3 + 3 = 6
3 + 3 = 6
3 + 3 = 6
3 + 3 = 6

There are two ways to achieve the following output in Nodejs

1 + 1 = 2
2 + 2 = 4
2 + 1 = 3
1 + 2 = 3

1. By wrapping setTimeout within a function to which the variables i and j are passed as arguments

    (function asynchronous(i, j) {
        setTimeout(() => {
            console.log(i+" + "+j+" = "+(i+j));
        }, 0)
    })(i, j)
   

2. By using let to declare the variables i and j which creates block scoped variables

    for(let i=1; i<3; i++) {
        for(let j=1; j<3; j++) {
            setTimeout(() => {
                console.log(i+" + "+j+" = "+(i+j));
            }, 0)
        }
    }