In Go, a defer statement defers the execution of a function until the surrounding function returns. Here's a simple example:
package main
import "fmt"
func main() {
fmt.Println("start")
defer fmt.Println("defer 1")
defer fmt.Println("defer 2")
fmt.Println("end")
}
In this example, the main function executes normally, but all deferred functions are executed in reverse order when the function exits. SO the output will be :
start
end
defer 2
defer 1
Now, adding defer keyword to JavaScript requires a lot of work. But that’s what I love about JS—it’s so versatile that you can implement features from other programming languages without needing to touch the compiler.
But first, why would we need this?
There are many useful use cases for a defer-like functionality in programming, such as:
- Resource cleanup: Clean up resources like file handles.
- Transactional operations: Roll back changes in case of an error, like reverting a database or state update.
- Logging: Add logging at the end of an operation.
- UI state management: Reset state after a process.
And the list goes on...
Now, let’s dive into the fun part: our implementation of defer in JavaScript.
class Deferer {
static stack = [];
static wrapped = false;
static defer(fn) {
this.stack.push(fn);
if (!this.wrapped) {
throw new Error("can't call defer wihtout wrapping the function with Deferer.wrapper")
}
}
static execute() {
while (this.stack.length > 0) {
const fn = this.stack.pop();
try {
fn();
} catch (err) {
throw new Error('Error in deferred function:', err);
}
}
this.wrapped = false;
}
static wrapper = (cp) => (...args) => {
if(this.wrapped) throw new Error("nested deferers are not supported");
this.wrapped = true;
try {
const v = cp(...args)
this.execute()
return v;
} finally {
this.wrapped = False;
}
}
}
const myDeferedFunction =Deferer.wrapper((a, b, c) => {
console.log("Start of function", a, b, c);
Deferer.defer(() => console.log("Deferred: Function 1"));
Deferer.defer(() => console.log("Deferred: Function 2"));
console.log("End of function", a, b, c);
});
myDeferedFunction(8,8,8)
Output:
Start of function 8 8 8
End of function 8 8 8
Deferred: Function 2
Deferred: Function 1
As expected, the deferred functions execute after the main function has completed, in reverse order.
Thanks,
Ahmed
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