What happens behind the scenes when a function is passed as a parameter in JavaScript and how does the engine recognize it as a callback?
What happens behind the scenes when a function is passed as a parameter in JavaScript and how does the engine recognize it as a callback?
To put it simply, functions in JavaScript are like any other value that can be passed around. This means we can pass numbers, strings, functions, and more as parameters.
Consider the following example:
// We will execute the function passed as a parameter
// This is essentially a callback that directly calls the function.
function call_function(f) {
return f();
}
var func = function(s) {console.log('func was called');}
var notAFunc = 42;
call_function(func); // func was called
call_function(notAFunc); // TypeError: number is not a function
The TypeError occurs because trying to call 42 as a function is like trying to do this:
42() // TypeError: number is not a function
In JavaScript, there is no built-in mechanism to check if a callback function is actually a function. Therefore, it is up to the programmer to ensure that the correct type of parameter is being passed.
As pointed out by Xufox in a comment, one way to check the type of the parameter is by using:
function call_function(f) {
if (typeof f === 'function') {
return f();
}
}
While this is a good practice, it is important to note that this type of validation is the responsibility of the programmer and not the JavaScript engine.
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