Question

var UsersMenu = function(){
    this.returnUsers = [];
    this.retrieve = function(posts){
        var temp = [];
        $.post("router.php", { "action": "getUsersMenu", "posts" : posts},
        function(data)
        {
            if(data.response){    
                for(var i=0; i<data.returnUsers.length; i++){
                    temp.push(data.returnUsers[i]);
                }
                this.returnUsers = temp; // i know what 'this' is incorrect
            }

        }, "json");
                alert(this.returnUsers);
    }
}

2 questions:
1. How to access to parent ‘this’ from jq object (returnUsers) ?
2. Why alert after jq post is calling before some alert in jq post ?

Answer 1

1 How to access to parent ‘this’ from jq object (returnUsers) ?

You could capture it in a closure:

var UsersMenu = function() {
    this.returnUsers = [];
    var self = this;
    this.retrieve = function(posts) {
        var temp = [];
        $.post("router.php", { "action": "getUsersMenu", "posts" : posts },
        function(data) {
            if(data.response) {
                for(var i = 0; i < data.returnUsers.length; i++) {
                    temp.push(data.returnUsers[i]);
                }
                self.returnUsers = temp;
            }
        }, "json");
    }
};

2 Why alert after jq post is calling before some alert in jq post ?

Because AJAX is asynchronous. The $.post method which sends an AJAX request returns immediately but the success callback handler is executed much later, when a response is received from the server. So you shouldn’t be putting the alert outside of this success handler. If you want to consume the results of an AJAX call this should happen only inside the callback which is when the result is available.