function template (template, data) {

    var result = template;

    for (var field in data) {
        var re = new RegExp( '\\$\\{' + field + '\\}', 'gi' );
        result = result.replace(re, data[field]);
    }

    return result;
}

*String template (String template, Object data)*

Usage: 
template('Hello <b>${name}!</b> (not ${name}?)', { name: 'Gandalf' })

Result: Hello <b>Gandalf!</b> (not Gandalf?)

Simplest template engine

template

JavaScript