Question

While searching the web, I’ve encountered a post that shows why the following example of dynamically generated methods does not work as planned:

// Create a new user object that accepts an object of properties
function User( properties ) {
    // Iterate through the properties of the object, and make sure
    // that it's properly scoped (as discussed previously)
    for ( var i in properties ) { (function(){
        // Create a new getter for the property
        this[ "get" + i ] = function() {
            return properties[i];
        };
        // Create a new setter for the property
        this[ "set" + i ] = function(val) {
            properties[i] = val;
        };
    })(); }
}

The reason for that is the anonymous function, which uses the “this” keyword is context of the “window”, instead of “User”.

1) Why does the this keyword in the anonymous function refers to “window instead of “User”?

2) Is there an accepted and common way to create “Dynamically Generated Methods”?

Thanks,

Joel

Answer 1

The reason that this refers to the window object, rather than User, is because this depends on the caller. In this case, the foreach contains an anonymous function that is immediately called. The caller will be considered to be the window object.

The reason it’s not working is because the code is poorly written. It would be a simple thing to pass both context and the i-variable to be scoped:

function User( properties ) {
    // Iterate through the properties of the object, and make sure
    // that it's properly scoped (as discussed previously)
    for ( var i in properties ) { (function(x){
        // Create a new getter for the property
        this[ "get" + x ] = function() {
            return properties[x];
        };
        // Create a new setter for the property
        this[ "set" + x ] = function(val) {
            properties[x] = val;
        };
    }).call(this, i); }
}