Working with undefined variables in Mongo shell

Question

I'm setting up a Mongo script to take variables by --eval, like so:

mongo mydb myscript.js --eval "parameter = 'value'"

However, I want the script to still work without requiring the --eval.

If this were browser-based JS, I'd just do a var internalParameter = window.parameter || null sort of thing to get around the ReferenceError thrown by checking for an undefined variable, but mongo doesn't have window. Is it possible to access variables through the global object in Mongo scripting, or do I just have to wrap this in a try/catch?

Answer 1

Now that I've learned a little more about common JavaScript idioms, I know you can also do:

var internalParameter = typeof parameter == "undefined" ? null : parameter

Answer 2

In myscript.js you can do:

var parameter = parameter || null;

If parameter is passed in via the --eval it will have that value, otherwise it will be null.

UPDATE

To do it without defining the input variable you can use the fact that this refers to the global object in the shell and do:

var internalParameter = this.parameter || null;

Answer 3

Yes. Your script could set a default like this:

var parameter = parameter || 'default';
print('parameter = ' + parameter);

And then it would be possible to pass parameters or leave the default:

% mongo script.js                                           
MongoDB shell version: 2.0.7
connecting to: localhost:27017/test
parameter = default

% mongo script.js --eval "var parameter = 'something else';"
MongoDB shell version: 2.0.7
connecting to: localhost:27017/test
parameter = something else

Answer 4

I did the following to solve a similar problem:

function set_param(name, default_val)
{
    if (typeof this[name] === "undefined")
        this[name] = default_val;
}

set_param("DAYS_BACK", 180);
set_param("BATCH_SIZE", 1000);

Now if either of the parameters is given with an --eval argument, MongoDB shell will use what was provided. Otherwise it will use the default supplied by the second argument to set_param(). The variables whose names were given as the first argument to set_param() are safe to use once set_param() is executed regardless of the use of --eval.