Throw/catch error when a variable is undefined using underscore

Question

I'd like to know if there is any solution to throw an error when a variable is not defined inside my template?

Example:

function hello() {
    var data = { foo: 'hello' };

    //data.bar is undefined, data.bar will be replaced by an empty string by underscore
    var render = _.template('<p><%= data.foo %><%= data.bar %></p>', {data: data}); 

    //Rendering okay? true|false
    //if (render...)

}

I'd like to throw an error if the variable data.boo is not defined after the call of the _.template function. Do you have any idea if it's possible to do something like that? I checked the documentation and didn't find anything interesting unfortunately...

Thanks

What you're trying to do just seems wrong and I think your code could be refactored not to need this behaviour. Can you summarise how do you want to handle the situation when data.foo is undefined? — mechanicalfish
– mechanicalfish, Commented Dec 2, 2013 at 20:20

nrabinowitz · Accepted Answer · 2013-12-02 20:35:05Z

2

For better or worse, Underscore will do this for you for top-level properties:

var data = { foo: 'hello' };
var rendered  = _.template('<p><%= foo %><%= bar %></p>', data);
// throws ReferenceError: bar is not defined

This is because Underscore templates use with, so top-level properties are treated like variables. If you want an error on a sub-object property, you'll need to add it in yourself, maybe using with as well:

var data = { foo: 'hello' };
// throw
var rendered  = _.template('<% with (data) { %><p><%= foo %><%= bar %></p><% } %>',
    { data: data });

Or you could use an explicit throw, e.g.

<% if (data.bar === undefined) throw "No bar!"; %>

at the beginning of your template. But really, why would you do it this way? If you know what you're checking for, far better to do it with an explicit pre-render check and deal with it there, and avoid using errors for control flow:

var rendered;
if (data.foo !== undefined) rendered = _.template("...", { data: data });
else {
    // do something else
}

answered Dec 2, 2013 at 20:35

nrabinowitz

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2 Comments

idbehold Over a year ago

This is a better answer than my own.

alexmngn Over a year ago

My template is actually an HTML with several dozen of variables, that's why I wanted to avoid to test all variables separately.

idbehold · Accepted Answer · 2013-12-02 20:41:38Z

0

Like @mechanicalfish said, what you're attempting to do seems incorrect. You should be validating the data outside of the template. In either case, here is what you asked for:

function hello() {
    var data = { foo: 'hello' };

    //data.bar is undefined, data.bar will be replaced by an empty string by underscore
    var render = _.template('<p><%= data.foo %><% ' +
      '  if (!data.bar) { ' +
      '    throw "data.bar is undefined" ' +
      '  } else { ' +
      '    print(data.bar) ' +
      '  } ' +
      ' %></p>', {data: data}); 

    //Rendering okay? true|false
    //if (render...)

}

answered Dec 2, 2013 at 20:41

idbehold

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Comments

Shreyas · Accepted Answer · 2013-12-02 20:13:13Z

-1

JavaScript has try/catch support.

function hello() {
    var data = { foo: 'hello' };

   try {

    //data.bar is undefined, data.bar will be replaced by an empty string by underscore
    var render = _.template('<p><%= data.foo %><%= data.bar %></p>', {data: data}); 
    } catch (e){
          throw "data.bar is undefined";
    }


}

answered Dec 2, 2013 at 20:13

Shreyas

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1 Comment

alexmngn Over a year ago

The render works perfectly even though the variable is undefined, it's my problem, there is no possibility to use try/catch there.

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