Just added Flow types to a project I'm working on and progressively adding types until I got to this error:

Cannot call JSON.parse with localStorage.getItem(...) bound to text because null or undefined [1] is inpatible with string [2]

This es from a expression:

const myVar = JSON.parse(localStorage.getItem('itemName'))

I understand why I get this error (except maybe the "bound to text" part), but couldn't find a way around it. I'd appreciate any help here!

Just added Flow types to a project I'm working on and progressively adding types until I got to this error:

Cannot call JSON.parse with localStorage.getItem(...) bound to text because null or undefined [1] is inpatible with string [2]

This es from a expression:

const myVar = JSON.parse(localStorage.getItem('itemName'))

I understand why I get this error (except maybe the "bound to text" part), but couldn't find a way around it. I'd appreciate any help here!

• javascript
• json

• because localStorage.getItem("someItem") can return a null value as well – Daksh M. Commented Oct 9, 2018 at 10:35
• Yes, this is the part I understand. The next step should be something like setting JSON.parse to accept null or undefined maybe? And how? – proko Commented Oct 9, 2018 at 10:38
• you can do an if statement or something like: localStorage.getItem("someItem") || {} – Daksh M. Commented Oct 9, 2018 at 10:39
• I tried something similar and unfortunately doesn't help. It just adds object literal is inpatible with string to the error. Problem is that JSON.parse only accepts a string. I guess I could wrap the getItem to a function and make sure it always returns a string but there should be a better solution. – proko Commented Oct 9, 2018 at 10:45
• localStorage.getItem("someItem") || '{}' wrap the object in string – Daksh M. Commented Oct 9, 2018 at 11:06

2 Answers 2

So, the function localStorage.getItem can return null values and flow wants you to tackle them before parsing it. As JSON.parse only takes a string, you can do the following:

localStorage.getItem("key") || '{}'

So, if it returns null. The empty object string is chosen, which JSON.parse can parse into an empty object.

Prefer using 'null' than '{}' as it parses to empty object

JSON.parse(localStorage.getItem("key") || 'null') // null

JSON.parse(localStorage.getItem("key") || '{}') // {} - empty object