So I'd like to destructure an object and have it throw an error if one of the keys didn't exist. I tried a try catch but it didn't work. And I'd like an alternative to just if (variable === undefined)

let obj = {test : "test"}

try {
	let { test, asdf } = obj 
  console.log("worked")
}
catch (error) {
	console.error(error)
}

So I'd like to destructure an object and have it throw an error if one of the keys didn't exist. I tried a try catch but it didn't work. And I'd like an alternative to just if (variable === undefined)

let obj = {test : "test"}

try {
	let { test, asdf } = obj 
  console.log("worked")
}
catch (error) {
	console.error(error)
}

• javascript
• ecmascript-6

• Have a look at this – Bergi Commented Apr 25, 2018 at 13:13

2 Answers 2

Use proxy to throw the error if a non-existent property is fetched

let obj = {
  test: "test"
};

try 
{
    let { test, asdf} = getProxy(obj);
    console.log("worked");
} 
catch (error) 
{
    console.error(error)
}

function getProxy(obj) {
    return new Proxy(obj, { //create new Proxy object
        get: function(obj, prop) { //define get trap
            if( prop in obj )
            {
               return obj[prop];
            }
            else
            {
                throw new Error("No such property exists"); //throw error if prop doesn't exists
            }            
        }
    });
}

Demo

let obj = {
  test: "test"
};

try 
{
    let { test, asdf} = getProxy(obj);
    console.log("worked");
} 
catch (error) 
{
    console.error(error)
}

function getProxy(obj) {
    return new Proxy(obj, {
        get: function(obj, prop) {
            if( prop in obj )
            {
               return obj[prop];
            }
            else
            {
                throw new Error("No such property exists");
            }            
        }
    });
}

The try-catch work for the runtime errors, catching exceptions or error you throw explicitly. Thus, in destructuring, there is no such error thrown when the matching key is not found. To check the existence you need to explicitly create a check for it. Something like this,

let obj = {test : "test"}
let { test, asdf } = obj 

if(test){
 console.log('worked');
} else {
 console.log('not worked');
}

if(asdf){
 console.log('worked');
} else {
 console.log('not worked');
}

This is because the destructuring works the same way as we assign the object value to another value like,

let obj = {test : "test"}
var test = obj.test;
var asdf = obj.asdf;

Here, doing obj.asdf will give you undefined in asdf and does not throw any exception. Thus, you cannot use try-catch for that.