Value check in nested javascript object (might not exist)

Question

Is there an elegant and concise way to avoid doing something like this to check a value deep within an object member hierarchy in javascript?

handlerInput.supportsDisplay = function() {
   return  this.requestEnvelope.context && 
           this.requestEnvelope.context.System && 
           this.requestEnvelope.context.System.device && 
           this.requestEnvelope.context.System.device.supportedInterfaces && 
           this.requestEnvelope.context.System.device.supportedInterfaces.Display;
}

Possible duplicate of Test for existence of nested JavaScript object key — t.niese
– t.niese, Commented Aug 26, 2018 at 17:47

Mikhail Burshteyn · Accepted Answer · 2018-08-26 17:38:58Z

1

There is currently no concise way to do it in plain JavaScript, unless you use a helper function or a third-party library.

There is a proposal (at Stage 1 as of August 2018) to add the ?. operator to JavaScript which does exactly what you want.

With that proposal accepted, you would be able to use ?. instead of . anywhere where a property might be missing, so your code would become:

// careful: this syntax is not available yet
var hasDisplay = handlerInput.requestEnvelope.context?.System?.device?.supportedInterfaces?.Display

edited Aug 26, 2018 at 17:38

answered Aug 26, 2018 at 17:32

Mikhail Burshteyn

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1 Comment

random_user_name Over a year ago

As a steward of StackOverflow, you should be voting to close as duplicate rather than providing a new answer to something that has been asked and answered thoroughly already.

seebiscuit · Accepted Answer · 2018-08-26 17:29:07Z

0

This is cheating, but you can use the _.get function from Lodash

_.get(validations, "path.to.nested.prop");

From the docs

Gets the value at path of object. If the resolved value is undefined, the defaultValue is returned in its place.

answered Aug 26, 2018 at 17:29

seebiscuit

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3 Comments

NAVIN Over a year ago

I guess he was asking if he could just check the nested value without using dots(a.b.c.d.e.f.g.h) and you did that only just with lodash.

seebiscuit Over a year ago

I disagree. He wants to do it in one line. That's what you _.get

random_user_name Over a year ago

I disagree that it's cheating. The LoDash library makes many, many things clearer and safer. I don't go anywhere without LoDash :)

Steven Spungin · Accepted Answer · 2018-08-28 01:56:10Z

I wrote an NPM module that allows you to query by a string path.

https://www.npmjs.com/package/js-path-resolver
http://www.twelvetone.tv/docs/developer-tools/npm-modules/js-path-resolver

Here's a codepen https://codepen.io/Flamenco/pen/xaVKjR/?editors=1111

For your use case:

import resolver from 'js-path-resolver'
const info = resolver(handlerInput, 'requestEnvelope.context.System.device.supportedInterface.Display', {onError:'continue'})
const hasDisplay = info.exists
const display = info.get()

In a nutshell, you split the string, then try to resolve the path, one string at a time. There are all sorts of issues implementing this, such as escaping dots, handling array indexes, etc, so using a library makes life much easier. In this library, you can also set, and delete the resolved path.

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