CODEWITHSUNDEEP
javascriptarraysnode.jsjsontypescript
Evorlor
Evorlor

Reputation: 7561

How can I check if my JSON field is defined?

I am parsing JSON and trying to access one of its fields, which is an array.

const myObj: MyObj = JSON.parse(myJson);
console.log(myObj.myArray); //SyntaxError: Unexpected end of JSON input
console.log(myObj.myArray == 'undefined'); //SyntaxError: Unexpected end of JSON input
console.log(myObj.myArray.length); //TypeError: Cannot read property 'length' of undefined
console.log(myObj.myArray !== null); //true

I have no problem iterating through my array and doing what I need with the data. But I need to check to make sure the myArray field is valid being iterating through it. When I try to do this, I get an error as shown above.

I am very very confused. Can anyone explain to me what is going on and how to fix it?

Upvotes: 1

Views: 106

Answers (4)

Rohìt Jíndal
Rohìt Jíndal

Reputation: 27202

You can check few things to verify that passed input is valid or not.

  • check for the data type : If type is already an object then no need to parse it again.
  • check for the truthy, i.e. if(myObj.myArray) { }
  • Looks like myJSON is returning an empty array. i.e. []

Hence, when trying to parse [] it returns "" and giving the below error.

SyntaxError: Unexpected end of JSON input

var myObj = JSON.parse([]);

console.log(myObj.myArray);

Upvotes: 1

Alberto Trindade Tavares
Alberto Trindade Tavares

Reputation: 10366

You can use the in operator to check whether a property is specified in an object prototype chain:

const myJson = '{"a": 2}';
const myObj = JSON.parse(myJson);

console.log('myArray' in myObj); // false
console.log('a' in myObj); // true

Alternatively, you can check whether the field is not undefined (if it is, it does not exist in the object returned by JSON.parse), and the safest way of doing this is by using the typeof operator:

const myJson = '{"a": 2}';
const myObj = JSON.parse(myJson);

console.log(typeof myObj.myArray !== 'undefined'); // false
console.log(typeof myObj.a !== 'undefined'); // true

A direct comparison against undefined (e.g., myObj.myArray !== undefined) should be avoided because undefined can be overridden or shadowed.

EDIT: As @OliveNi pointed out, the usage of the in operator is dangerous because it goes through the entire prototype chain. For a safer check (as an alternative to undefined check), you can use hasOwnProperty as exemplified in his answer.

Upvotes: 1

Joe
Joe

Reputation: 42627

If you are concerned about existence, you can simply check

if (myObj.myArray) {

If you want to confirm existence and confirm it's an array

if (myObj.myArray && Array.isArray(myObj.myArray) {

Upvotes: 1

Oliver Ni
Oliver Ni

Reputation: 2664

Be careful while using in, as it searches the entire prototype chain.

const myJson = '{"a": 2}';
const myObj = JSON.parse(myJson);

console.log('constructor' in myObj); // true (BAD)

Instead, try using .hasOwnProperty(), like so:

const myJson = '{"a": 2}';
const myObj = JSON.parse(myJson);

console.log(myObj.hasOwnProperty("a")); // true
console.log(myObj.hasOwnProperty("constructor")); // false

Upvotes: 1

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