`prop-types` rule reports for Array.prototype.length is missing

[ashleahhill]

h2. Expected

When a propType React.PropTypes.arrayOf(<something>), you shouldn't be prompted to add
length by eslint.

h2. What happens

You get '.length' is missing in props validation (react/prop-types) from eslint

h2. Example

class MyClass extends React.Component {
    static propTypes = {
        results: PropTypes.arrayOf(
            PropTypes.shape({
                items: PropTypes.arrayOf(PropTypes.shape(<Shape definition>))
            })
        )
    };

    get hasItems() {
        this.props.results && this.props.results.items && this.props.results.items.length > 0; 
    }
}

Results in this:
'results.items.length' is missing in props validation (react/prop-types)

[guidobouman]

@ljharb Any pointers as to where it would be smart to implement the filtering of default methods & properties? Somewhere in https://github.com/yannickcr/eslint-plugin-react/blob/master/lib/rules/prop-types.js ofcourse. But at what stage might this be best to implement? reportUndeclaredPropTypes() & Program:exit() seem a little late in the chain.

[karissame]

I have a similar issue- is there any way to get the length of an array in your component's props?

[ljharb]

@karissame I built withShape in airbnb-prop-types specifically so you could do withShape(PropTypes.array, { length: PropTypes.number }) or similar, since React.PropTypes makes that impossible.

That said, the plugin should definitely exempt anything on Array.prototype when PropTypes.array or arrayOf is used.