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Components and Advanced JSX

Put Logic in a Render Function

A render() function must have a return statement. However, that isn't all that it can have.

A render() function can also be a fine place to put simple calculations that need to happen right before a component renders. Here's an example of some calculations inside of a render function:

class Random extends React.Component { render() { // First, some logic that must happen // before rendering: const n = Math.floor(Math.random() * 10 + 1); // Next, a return statement // using that logic: return <h1>The number is {n}!</h1>; } }

Watch out for this common mistake:

class Random extends React.Component { // This should be in the render function: const n = Math.floor(Math.random() * 10 + 1); render() { return <h1>The number is {n}!</h1>; } };

In the above example, the line with the const n declaration will cause a syntax error, as is it should not be part of the class declaration itself, but should occur in a method like render().

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