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This is one of the places which two ways data binding that's used in many new frameworks in JavaScript will work.

So if you use Angular, React or any other frameworks which do two ways data binding, this issue is simply fixed, so in easy word, your result is undefined at the first stage, so you have got result = undefined before you recieve the data, then as soon as you get the result, it will updated and get assigned to the new value which is respond of your Ajax call...

But how you can do it in pure javascript or jQuery for example as you asked in this question?

You can use a callback, promise and recently observable to handle it for you, for example in promises we have some function like success() or then() which will be executed when your data is ready for you, same with callback or subscribe function on observable.

For example in your case which you are using jQuery, you can do something like this:

$(document).ready(function(){
    function foo() {
        $.ajax({url: "api/data", success: function(data){
            fooDone(data); //after we have data, we pass it to fooDone
        }});
    };

    function fooDone(data) {
        console.log(data); //fooDone has the data and console.log it
    };
    
    foo(); //call happens here
});

For more information study about promises and observables which are newer ways to do this async stuffs.