You can’t assign a runtime default inside a TypeScript interface. An interface describes an object’s shape; put defaults in executable code where you read options or create an object. For optional settings, choose a fallback that preserves intentional values such as false and 0.
Can a TypeScript interface have default values?
No. An interface is a type-level contract, not executable code, so a declaration such as theme: "light" describes a required property; it does not initialize that property. The TypeScript documentation describes interfaces as defining object shapes and shows defaults being applied by the code that consumes the object. See the current Object Types handbook.
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Mark properties with ? when callers may omit them, then provide defaults in a function, normalizer, factory, or constructor. For example:
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theme?: "light" | "dark";
compact?: boolean;
pageSize?: number;
}With strictNullChecks, reading an optional property means handling the possibility of undefined. Optionality permits a caller to leave out a value; it does not guarantee that the value will be present later.
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1. Use explicit fallback checks when reading options
For one or two defaults used in a particular function, check for undefined where the values are needed:
function describe(options: DisplayOptions) {
const theme = options.theme === undefined ? "light" : options.theme;
const compact = options.compact === undefined ? false : options.compact;
return { theme, compact };
}This form defaults only when a property is missing or explicitly undefined. It preserves false, 0, and an empty string if those are valid values for the property. The handbook’s optional-property examples likewise check for undefined before using a value.
?? is a compact alternative when both null and undefined should trigger the default: const pageSize = options.pageSize ?? 20;. Use === undefined if explicit null should be treated differently. Avoid || when a falsy value such as false or 0 is meaningful, because it replaces that value too.
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2. Set defaults while destructuring a function parameter
When a function consumes an options object, destructuring defaults make the values available directly inside the function:
function render({
theme = "light",
compact = false,
pageSize = 20,
}: DisplayOptions) {
return { theme, compact, pageSize };
}A destructuring default applies when that property is absent or its value is undefined; it does not apply to null. This is useful when the defaults belong to one function rather than to a shared configuration policy. The TypeScript Object Types handbook demonstrates typed destructured parameters with defaults.
If callers may omit the entire options object, default the parameter to {}. This works here because every property in DisplayOptions is optional:
function render({ theme = "light" }: DisplayOptions = {}) {
return theme;
}3. Merge caller options with a reusable defaults object
When several parts of a program need the same defaults, keep them in one object and normalize incoming options in a single place:
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pageSize: 20,
} satisfies Required<DisplayOptions>;
function normalizeDisplayOptions(options: DisplayOptions) {
return { ...displayDefaults, ...options };
}In an object spread, later properties take precedence, so values supplied by the caller override the defaults. The result contains all three settings unless the input explicitly supplies undefined for a property, which would overwrite that default. If that input is possible and should count as missing, normalize it with explicit fallback logic instead.
This merge is shallow. If a setting contains a nested object and callers can provide only part of it, spread the nested defaults and nested input deliberately rather than expecting the outer spread to combine them. The satisfies operator checks that the defaults meet the required shape while preserving the expression’s inferred type; it requires TypeScript 4.9 or later. For earlier versions, use a suitable type annotation instead. The Utility Types documentation covers utilities such as Required<T>; the spread-and-normalization recipe is an application pattern.
4. Use Partial input and a complete output type
Partial<T> is useful when an input is intentionally incomplete. Define the completed settings separately so code after normalization can rely on every property:
interface DisplaySettings {
theme: "light" | "dark";
compact: boolean;
pageSize: number;
}
type DisplaySettingsInput = Partial<DisplaySettings>;
function makeDisplaySettings(input: DisplaySettingsInput): DisplaySettings {
return {
theme: input.theme ?? "light",
compact: input.compact ?? false,
pageSize: input.pageSize ?? 20,
};
}The input type allows any subset of the settings, while the return type promises all of them. Partial<T> and Required<T> change what the type checker permits; they do not create values or fill properties at runtime. The Utility Types handbook defines these transformations.
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For plain objects, use a factory
A factory is a clear creation boundary when multiple callers need a complete object:
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function createDisplayOptions(
input: DisplayOptions = {},
): Required<DisplayOptions> {
return {
theme: input.theme ?? "light",
compact: input.compact ?? false,
pageSize: input.pageSize ?? 20,
};
}Callers can pass partial options—or omit them—and the returned object has all three properties. Because this factory uses ??, explicit null also selects the default if the input type is widened to allow null.
For instances, initialize in the class
When settings belong to a class instance, assign them in the constructor or use class-field initializers. The interface can describe the resulting instance shape, but the constructor or field initializer performs the actual work.
Which technique should you use?
| Situation | Good starting point | Why |
|---|---|---|
| One or two values are used by one function | Explicit fallback or parameter destructuring | Keeps each default close to where it is consumed. |
| Many optional settings are reused in several places | Defaults object plus normalization function | Centralizes the policy and produces a completed configuration. |
| Input is intentionally incomplete, but internal code needs every field | Partial<T> input and complete output type |
Makes the boundary between partial input and normalized settings explicit. |
| A value is created as a domain object or instance | Factory or constructor | Places initialization at the object-creation boundary. |
Before choosing, decide where the policy belongs, whether the whole object can be omitted, and whether explicit null or undefined should count as a supplied value. Those choices determine whether a destructuring default, ??, an undefined check, or a normalization function expresses the intended behavior.
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Quick Recap
Common mistakes to avoid
- Putting an initializer in an interface. Interfaces describe types; executable code must create the value.
- Assuming an optional property is always present. Handle possible
undefinedwhen reading it, particularly withstrictNullChecks. - Using
||for every fallback. It can discard intentional values such asfalseand0. - Expecting
Partial<T>to fill in defaults. It changes the type, not the runtime object. - Expecting spread to deep-merge nested settings. Add explicit handling for nested objects.
- Applying shared defaults separately in multiple consumers. Normalize at one boundary when multiple parts of the program depend on the same completed configuration.
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