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Make a prop optional in TypeScript
Describe the component’s props with an object type declared using either type or interface. Put ? after the property name to make it optional at the call site. For example, size?: number lets a caller render an Avatar without providing size, while person remains required:
type AvatarProps = {
person: { name: string };
size?: number;
};
function Avatar({ person, size = 100 }: AvatarProps) {
return <img alt={person.name} width={size} height={size} />;
}TypeScript checks these props and can provide inline editor documentation. The ? changes the caller contract; it does not, by itself, choose a value for the component to use.
Set a fallback for an omitted prop
For a function component, put the fallback on the destructured property in the parameter, as in size = 100. The component then uses 100 when size is absent or explicitly undefined. The caller can omit the prop entirely:
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<Avatar person={{ name: "Ada" }} />Or pass undefined and get the same fallback:
<Avatar person={{ name: "Ada" }} size={undefined} />The TypeScript Handbook recommends putting the fallback on the optional destructured property itself. Keep destructuring straightforward; deeply nested destructuring mixed with defaults and type annotations can be harder to follow. See the TypeScript Handbook’s destructuring guidance.
Know what does—and does not—trigger the default
A JavaScript parameter default applies when the value is missing or undefined, not whenever it is falsy. React’s default-value example makes these distinctions explicit:
Value supplied for size |
Value used inside the component |
|---|---|
| Prop omitted | 100 |
undefined |
100 |
null |
null |
0 |
0 |
Choose the prop type and fallback to match the component’s contract. If null is a meaningful explicit value, preserve and handle it as such. If null should also trigger a fallback, implement that rule separately; a destructuring default will not do it. Likewise, don’t use a truthiness check when zero is valid.
Choose the right defaulting approach for your React version
For modern function components, use parameter defaults rather than Component.defaultProps. React 19 removed defaultProps for function components in favor of ES6 default parameters; class components continue to support defaultProps. The React 19 Upgrade Guide, dated April 25, 2024, illustrates an optional TypeScript prop such as text?: string paired with a destructuring default such as text = 'Hello, world!'.
If you are maintaining an older project or upgrading, check its React and React type-definition versions before changing existing code. React’s upgrade guidance tells TypeScript users to update @types/react and @types/react-dom as part of a React 19 upgrade.
Read props without changing them
A function component receives one props object and can read it directly or destructure the values it needs. Props are read-only snapshots for a render: do not mutate an optional prop to make an interactive value change. Use state for values that need to change over time.
JSX spread can forward all props, but forwarding everything widely can make component boundaries less clear. React recommends restraint; if a wrapper mostly passes props through, consider whether the component structure should change or whether nested JSX using children would express the relationship better. See React’s guide to passing props.
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