Handlebars: Access Has Been Denied To Resolve The Property “From” Because It Is Not An “Own Property” Of Its Parent

When you’re working with Handlebars, such error messages often pop up when the engine is trying to access a certain property from an object but fails to do so. When we talk about an “own” property in JavaScript and thus in Handlebars.js layout engines, we refer to a property that is directly attached to an object and not inherited through a prototype chain.

Let’s think about having an object

user

in your data context:

var user = { 
"name": "John", 
"email": "john@example.com",
"address": {
    "street": "123 Main St"
    } 
};

If for example, you try to access the “street” property directly from the

user

object like so:

{{user.street}}

, Handlebars won’t be able to find it because “street” is nested within the “address” object and is therefore not an “own” property of the

user

object.

The right way to access nested properties is by traversing the object tree:

{{user.address.street}}

But if a property named “from” does not exist on the top-level user object or in any nested objects accessible from the user, Handlebars will raise an Access has been denied error, as it cannot resolve the non-existing property “from”.

If you need more details regarding ‘Own Property’ understanding, these two references might help you: MDN web docs – Object.prototype.hasOwnProperty() & Handlebars.js Guide.
In JavaScript, every object has some properties and methods. These properties are either the object’s own properties or those that the object inherits from its prototype chain. In order to understand the error message “Access Has Been Denied To Resolve The Property ‘From’ Because It Is Not An ‘Own Property’ Of Its Parent” while working with Handlebars, one needs to have a good understanding of own properties in JavaScript.

When we say an object has an “own property”, it refers to the properties defined directly on the object itself and not inherited through a prototype chain.

Let’s look at an example:

let objA = {
  name: "John",
};

console.log(objA.hasOwnProperty("name")); // This will return true
console.log(objA.hasOwnProperty("toString")) ;// This will return false

Here,

name

is an own property of

objA

, but

toString

is not, even though you can access it through

objA.toString()

. The

toString()

function is inherited from

Object.prototype

, so it is not an ‘own property’ of

objA

.

Coming back to the error you’re experiencing in Handlebars, this issue most commonly happens when your template is trying to access a property that is not an own property of the input object. Handlebars, by design, only allows access to an object’s own properties.

For instance, if you’re providing an array as input to Handlebars and you’re trying to use native array properties like

length

or

concat()

within a block, Handlebars would complain since these are not “own properties” of the array, they’re properties of

Array.prototype

.

Here is what your code might look like:

let data = ['one', 'two', 'three'],
    templateSource = '{{#each this}}{{length}}{{/each}}';  
    
let template = Handlebars.compile(templateSource);
let result = template(data);

In this case, changing how you structure your data to ensure all necessary values are own properties should fix the issue. Here’s an illustrative example:

let data = {
  items: ['one', 'two', 'three'],
  length: 3   
},  
templateSource = '{{#each items}}{{../length}}{{/each}}';
      
let template = Handlebars.compile(templateSource);
let result = template(data);

In summary, understanding ‘own properties’ in JavaScript is key to properly using and debugging Handlebars templates. It’s crucial to make sure that any data elements your Handlebars templates need to refer to are passed in as own properties of the context object given to your compiled template function. If accessing properties of JavaScript’s built-in objects (like

Array

or

Date

) within Handlebars templates is essential for your application, consider transforming such data and make sure all required properties are included in your Handlebars context.

Also, be aware of the Best Practices mentioned in the Handlebars documentation regarding usage of JavaScript’s built-in properties.

When dealing with more complex data structures, making use of libraries to manage immutability and enforce your data being composed purely of own properties can prove useful. Libraries like lodash’s

.cloneDeep()

or the Immutable.js library provide ways to manage this effectively.Let’s delve deep into how properties are accessed in Handlebars. To set the stage, Handlebars is a template engine that helps to dynamically generate HTML pages using data. In essence, it binds data to {{placeholder}} tags within your HTML files.

Typically, if you’ve received an error message similar to “Access has been denied to resolve the property ‘from’ because it is not an own property of its parent” while using Handlebars, this signifies that there’s a reference to a property named ‘from’ that isn’t directly a property (or ‘own property’, as often phrased) of the object being evaluated.

In JavaScript, an ‘own property’ refers to a property attached directly to an object, as differentiated from a property being attained down the object’s prototype chain. If Handlebars can’t find a property on the immediate data context, it will traverse up the lineage of parent contexts to unearth it.

Consider, as an illustrative example, the following data structure and Handlebar syntax:

var context = {
parent : {
    child: {
    from: "I'm from child"
    }
}
};

Your Handlebars syntax may be referencing the ‘from’ attribute like this:

Value: {{from}}!

Given the above, you’d obtain the error message in question because ‘from’ isn’t an own property of the ‘context’ object. The fix for such an issue would be adjusting the code to read:

Value: {{parent.child.from}}!

A critical point to note is that from Handlebars version 4.x.x onwards, access to prototype properties was barred due to security reasons. If you’re attempting to access inherited properties utilizing the framework, you’ll encounter the error under discussion.

To mitigate this scenario, one could define all necessary properties directly on the object itself rather than relying on inheritance. Alternatively, another approach could be to flatten your object using Object.assign() or underscore’s _.extend(). For further information on why this restriction was put in place, you might want to refer to the related GitHub issue.

Web development is replete with unique challenges. Encountering this idiosyncrasy when working with Handlebars specifically doesn’t dissuade from its overarching benefits. With a versatile template engine, controlling the rendered HTML becomes smoother, which culminates in more maintainable and extensible code, thereby facilitating speedy web application development.If you’re coding with Handlebars, you might come across an “Access Has Been Denied To Resolve The Property ‘From’ Because It Is Not An ‘Own Property” Of Its Parent” error. This is typically because of the way Handlebars restricts access to only an object’s own properties. When your program tries to reference a property that isn’t a owned by the object itself, it’s not permitted and hence the error message.

Let’s illustrate this concept through a simple bit of code: