What is the second argument that can optionally be passed to setState and what is its purpose?

Question

Q:

What is the second argument that can optionally be passed to setState and what is its purpose?

Answer 1

A callback work which will be conjured when setState has completed and the part is re-rendered. 
Something that is not talked about a great deal is that setState is asynchronous, which is the reason it takes in a moment callback function. Ordinarily it's best to utilize another lifecycle strategy instead of depending on this callback function, however, it's great to know it exists.

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Class Training extends Course { this.state = { sampleItem: 'learn', } handleChange = (event) => { console.log(this.state.sampleItem) this.setState({ sampleItem: event.target.value      //event.target.value = Welcome }, () => console.log(this.state.sampleItem)) };

Output:

Learn

Welcome