javascript - Make sure variable is array

I want to run map() on a variable that might either be a single object or an array. However, I can't do this if the variable is an object as that throws an error.

I tried using the spread operator to make sure the variable turns into an array but apparently that doesn't work either:

// items can either be an object or an array of objects
renderedItems = [...items]
    .filter(item => !!item)
    .map((item, index) => _this.renderItem(item, index));

How can I make sure the variable I'm doing a filter() and map() on is an array? I heard Array.isArray isn't fully supported yet.