React: checker is not a function

Question

I\'m getting this weird warning message in the console for my React app.

Warning: Failed propType: checker is not a function Check the render method of <

Answer 1

The real explanation behind this issue is that react expects a PropType which is technically a function, aka checker. Examples for checker functions are:

PropTypes.array
PropTypes.shape({...})
PropTypes.bool

You got the idea... So what happens here is whenever you pass in something which isn't a checker, for example an undefined, react won't know what to do with this and so will complain:

checker is not a function

In my case, the cause was misspelling of the checker functions:

PropTypes.shape({
    cats: PropTypes.Array // this is undefined
})

The correct version should be:

PropTypes.shape({
    cats: PropTypes.array // lowercase is correct!
})