CODEWITHSUNDEEP
javascriptundefined
Zerium
Zerium

Reputation: 17333

Casting all properties of undefined variables and objects to 0 in JavaScript

I have a section of code like the following:

var arr = [];

// bit further down in the code
arr[arr.length - 1].id + 1

Evidently, if arr is empty, then arr[arr.length - 1] would return me undefined.

Now I know that I can cast that to 0 with a simple or statement like so:

arr[arr.length - 1] || 0; // will return 0 if arr[arr.length - 1] is undefined

However, I wish to either return arr[arr.length - 1].id, or 0, if arr[arr.length - 1] is undefined.

Is there a shorthand notation that I can use to solve this issue? I have tried:

arr[arr.length - 1].id || 0

However, this returns me an error Cannot read property 'id' of undefined, which undoubtedly would make sense in this context.

Upvotes: 0

Views: 49

Answers (2)

Cezar Augusto
Cezar Augusto

Reputation: 9852

That's because it's looking for .id, which doesn't exist because the engine is looking for a property of a unknown object. You must first check is arr is defined, then proceed the operation:

var arr = [];
// It will check for ID only if arr exists and output the result.
// Otherwise value is 0
(arr && arr[arr.length - 1].id) || 0

Upvotes: 0

Pointy
Pointy

Reputation: 413702

You can check the length first:

arr.length && (arr[arr.length - 1].id + 1) || 0;

That will evaluate to the last element's "id" value plus 1, or else zero if the array is empty.

Upvotes: 2

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