JavaScript Arrays - removing items

[s2t2]

Translate this react code into a simple example:

   handleCategoryCheck(changeEvent){
        var category = changeEvent.target.value
        console.log("CHECK CATEGORY:", category)

        var userCategories = this.state.userCategories
        var categoryIndex = userCategories.indexOf(category) // will be -1 if item not in array
        if (categoryIndex >= 0 ) {
            userCategories.splice(categoryIndex, 1) // remove 1 item from array at the given position
        } else {
            userCategories.push(category)
        }

        console.log("CATEGORIES:", userCategories)
        this.setState({userCategories: userCategories}) // this is updating state but why not triggering a re-render?
    }