为什么不能将字符串作为数据参数传递给Blaze.renderWithData（）？

Question

I'm creating a Blaze View with the following call:

Blaze.renderWithData(Template.my_template, myobj._id, html.node());

my_template looks like this:

<template name="my_template">
  {{> Template.dynamic template=whichTemplate data=sdata}}
</template>

The sdata helper looks like this:

sdata: function() {
  return Doodads.findOne({_id: this});
}

Which fails with the following error:

Exception in template helper: TypeError: selKey.substr is not a function

If I do console.log(this) within that helper, I get:

String {0: "N", 1: "7", 2: "j", 3: "o", 4: "y", 5: "g", 6: "w", 7: "P", 8: "g", 9: "e", 10: "s", 11: "R", 12: "f", 13: "w", 14: "q", 15: "o", 16: "7", length: 17, [[PrimitiveValue]]: "N7joygwPgesRfwqo7"}

To correct the problem, I can change the findOne() line to:

  return Doodads.findOne({_id: ""+this});

Which works as expected.

---

I am posting this question to help myself (and others) understand what is going on here.

Mainly I want to know:

• Why is there difference between using a String{...} object and a regular string for this call (ie, why doesn't the selector key object have a substr() method when I'm using a different/weird selector value)?
• Why does the _id property show up as this weird String{...} object as the template data this when the _id property is usually just a string? Or is it not usually and I've just never noticed?

I've solved the problem with the ""+this hack, but is there a more proper way to do this?

Answer 1

If your data is created from some other source than a Meteor subscription, it could be that your _id is a MongoDB ObjectId, which is not a string.

Have a look in the Mongo shell, if you do a .findOne on the myobj object and it shows up looking something like

"_id" : ObjectId("57cd946429bca10300f0fd55")

Answer 2

what is wrong?

According to the Blaze API docs , the signature is:

Blaze.renderWithData(templateOrView, data, parentNode, [nextNode], [parentView])

data Object or Function

The data context to use, or a function returning a data context. If a function is provided, it will be reactively re-run.

The data context, in your case, should be an object which includes a whichTemplate property and some id that you can use in your helper.

Blaze.renderWithData(Template.my_template, {
  _id: myId, 
  whichTemplate: someTemplateName
}, someNode);

or, instead of a data object, pass a function that will be reactively re-run.

With this setup, your helper code should be something like:

sdata: function() {
  return Doodads.findOne(this._id);
}

source of the error you were seeing

If you are interested in the cause of the error you were getting, here's what is going on:

In blaze, the countext is bound to the template helper's this reference using Function.protptype.apply(thisArg, [argsArray]) (in this line ).

According to the EcmaScript standard, if thisArg is a primitive, it will be boxed.

thisArg

The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed .

(source: MDN )

This causes a String object, which does not have a substr method, be transferred to the MiniMongo.Collection 's find() method, which fails here .