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Check if a prop was passed to a component in React:

1. Compares prop to undefined .
2. If the prop is equal to undefined , then it is not passed to the component.
3. Otherwise, it is passed to the component.

const Button = ({withIcon}) => {
  if (withIcon !== undefined) {
    console.log('prop was passed');
  } else {
    console.log('prop was NOT passed');
  }

  return <button>Click {withIcon ? '▶️' : null}</button>;
};

export default function App() {
  return (
    <div>
      <Button />
    </div>
  );
}

The Button component takes withIcona property.

If the prop is not passed to the component, its value will be undefined.

In our JSX code, we can use the ternary operator to check if a prop was passed to a component.

The ternary operator is very similar to an if/else statement.

It checks if the value to the left of the question mark is true, and if so, the operator returns the value to the left of the colon, otherwise it returns the value to the right.

We can also use the logical AND &&operator to check if a prop is passed in our JSX code.

const Button = ({withIcon}) => {
  if (withIcon !== undefined) {
    console.log('prop was passed');
  } else {
    console.log('prop was NOT passed');
  }

  return <button>Click {withIcon && '▶️'}</button>;
};

The logical AND &&operator returns the value on the right if the value on the left is true.

We can use this approach because boolean values, null , and undefined are ignored. They are not rendered at all.

None of the following JSX expressions render anything.

<div />

<div></div>

<div>{false}</div>

<div>{null}</div>

<div>{undefined}</div>

<div>{true}</div>

If we need to check if a prop was passed to a component outside of its JSX code, compare the prop to undefined .

const Button = ({withIcon}) => {
  if (withIcon !== undefined) {
    console.log('prop was passed');
  } else {
    console.log('prop was NOT passed');
  }

  return <button>Click {withIcon && '▶️'}</button>;
};

If the prop is equal to undefined , then it is not passed to the component, unless the parent component explicitly passes an undefined value for the prop, which is pretty much the same.