React: onClick handler is getting called on every render?

I am learning React, and followed this tutorial to create a simple Tic-Tac-Toe game that you can view here in CodePen.

I am confused about how the arrow function works in the onClick property of the Square component that is being returned inside of the renderSquare function of the Board component: onClick={() => this.props.onClick(i)}. And also again similarly the Game component where I have onClick={ (i) => this.handleClick(i)}. I assumed I could write it without the arrow function (just like onClick={this.handleClick(i)}) but this breaks the game.

Solution 1:

onClick expects a function. An arrow function does not have its own this; the this value of the enclosing execution context is used. Arrow function is a replacement for the following

onClick={this.handleClick.bind(this,i)}

It doesn't work when you run it like

onClick={this.handleClick(i)} 

because in this case it will call a function and that will pass a return value that will be evaluate everytime render is called. So if you are doing somethings in the onClick function that causes a rerender for instance setState you app will go in an endless loop. Thus onClick needs a function and not a value so unless you are returning a function from the onClick handler you should not directly call it.

Arrow function above performs the role of binding the parameter to the function