Handlebars ‘in’ block helper

README.md

Explanation

Some APIs only return a certain property if it’s supported in that context. This makes using the traditional {{#if}} block helper difficult, even when passing in the little known includeZero=true; the property value might be null, and in some edge cases even undefined.

in.js

define('shared/handlebars/helpers/in', ['handlebars'], function(Handlebars)
{
    var inHelper = function(attribute, options)
    {
        if(arguments.length < 1)
            throw new Error('Handlerbars Helper “in” needs an “attribute” parameter');

        var result = attribute in options.data.root;
        return options[result ? 'fn' : 'inverse'](this);
    };

    Handlebars.registerHelper('in', inHelper);
    return inHelper;
});

usage.hbs

{{#in 'someAttribute'}}
    {{! Render handling for someAttribute here, even if its initial value is falsy }}
{{else}}
    {{! Explain why someAttribute isn’t available in this context, or skip this entire else clause }}
{{/in}}