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Q:

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

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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

React JS
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