What is really happening when I call a function?

Question

This might be a simple question but I've been thinking lately. I've tried researching for the answer but I have yet to find a satisfactory one.
Basically, what's happening behind the scene when I call a function? Say:-

function sayHello(){
    console.log('hello');
}
sayHello(); //what's happening here?

I know that doing sayHello.call(); or sayHello.apply(); is the same thing as doing sayHello(); but is there any more information on what's happening underneath or is it behind the mysterious native code?

Answer 1

Here are some of the things the interpreter does to make a JS function call:

• A new scope object is created. An arguments object is created and put into the scope object with any arguments passed to the function in it.
• Any local variables in the new function are put into the scope object.
• A reference to the next line of code is pushed onto the execution stack (so the interpreter knows where to go when the function returns).
• The this pointer is set as appropriate.
• Execution is transferred to the code of the function.

This is managed by the internals of the JS interpreter (one of its many jobs) which is likely native code.

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If you want to call function B() anytime function A() is called, you can replace function A() with your own proxy which calls function B() and then calls the original A() .