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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchUse TypeScript’s any only when you deliberately need to opt out of type checking—for example, as a temporary bridge during a JavaScript migration or when working with code whose types are unavailable. If a value’s shape is simply unknown, start with unknown and narrow it before using it. Enable noImplicitAny or strict to catch inferred any; neither setting bans explicit any annotations.
What any does
A value typed any can be assigned to other types and used with arbitrary property access, function calls, and operations without the compiler checking whether they are valid. Values derived from it can also become any, allowing the unchecked type to spread through your code.
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For example, this compiles even though the parsed object’s count property is a string rather than a number:
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function readExternalValue(): any {
return JSON.parse('{"count":"not a number"}');
}
const count: number = readExternalValue();The assignment is accepted because any opts out of the relevant check; it does not establish that the runtime value is a number. TypeScript’s Handbook describes any as a way to work with existing JavaScript while opting in or out of type checking during compilation.
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Type annotations are erased at runtime. They do not validate JSON or other external input. If your program needs to trust a value from outside the type-checked code, validate it at runtime before relying on its shape.
Choose between any, unknown, and noImplicitAny
| Situation | Better starting point | Reason |
|---|---|---|
| The value’s type is not known yet | unknown |
It can hold any value, but must be narrowed before use as a specific type. |
| The value is passed through without being inspected | unknown, in most cases |
It represents uncertainty without permitting unchecked operations. |
| A JavaScript migration needs a temporary bridge | Explicit, narrowly scoped any |
It can help migration proceed, but that location has no type safety and should be revisited. |
The compiler infers any because it lacks enough information |
noImplicitAny or strict |
These settings report inferred any; they do not prohibit explicit annotations. |
| A callback’s return value is intentionally ignored | void |
TypeScript’s Do’s and Don’ts recommends void rather than any for this case. |
Why unknown is safer for uncertain values
unknown lets you hold a value without claiming to know its type. Before using it as a number, string, or other specific type, first perform a check that narrows it. For example:
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- TypeScript implements a superset of syntax for strictly typed development, facilitating deep static analysis and enhanced development environment integration. The compiler translates source into standard script formats, ensuring parity across any runtime.
- TypeScript is ideal for front-end developers, full-stack engineers, and software architects who build large-scale web applications. It serves those looking to improve code excellence, reduce bugs through static checking, and maintain complex projects more.
- Lightweight, Classic fit, Double-needle sleeve and bottom hem
function readExternalValue(): unknown {
return JSON.parse('{"count":3}');
}
const value = readExternalValue();
if (typeof value === "number") {
// value is narrowed to number here.
}This illustrates narrowing, not complete JSON validation: the example returns an object, so the number check does not establish that the object has a numeric count property. Validate the actual structure your program expects.
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noImplicitAny tells the compiler to report places where it would otherwise infer any. A developer can still explicitly write : any. The TSConfig reference documents the option; the broader strict family enables it along with other strict checks.
To enable the broader set in tsconfig.json:
{
"compilerOptions": {
"strict": true
}
}You can enable noImplicitAny directly instead if you want that inference check without turning on the full strict family. If your team also wants to discourage explicit any, establish a separate linting or code-review policy; compiler configuration alone does not do that.
When using any is reasonable
- Gradual JavaScript migration: use it as temporary scaffolding where modeling a value would otherwise block conversion. Keep each use narrow and replace it as the value becomes better understood. TypeScript’s migration guidance discusses adopting stricter checks during conversion and acknowledges that
anylocations lack type safety. - Unavailable type information: a third-party library or existing JavaScript code may not provide usable types. The Handbook describes
anyas an option when type information is unavailable or an annotation would take inappropriate effort. Prefer a narrow boundary over letting it spread through unrelated code. - Deliberately unchecked interoperation: if you intentionally need to interact with a value without TypeScript checking those operations,
anyprovides that escape hatch. Make the loss of checking explicit rather than treating it as an ordinary default.
How to reduce any during a migration
- Turn on
noImplicitAnyorstrict. This exposes places where the compiler inferredanybecause it lacked information. - Keep explicit migration annotations localized. Use
anyonly at points where conversion needs a temporary bridge, rather than returning it from broadly used functions or passing it through many layers. - Replace each annotation as the value becomes understood. Give known values appropriate types; use
unknownwhen the value is still uncertain and narrow it before operations that depend on its shape. - Revisit remaining explicit uses. Compiler settings catch inference problems, not explicit annotations, so review those annotations through code review or a separate linting policy.
The declaration-file guidance advises avoiding any except during migration and using unknown when a type is not known or a value will pass through without interaction.
Handle caught errors as unknown
JavaScript permits throwing values of any type, so a caught value is not guaranteed to be an Error. TypeScript 4.4 added useUnknownInCatchVariables, which makes catch variables default to unknown; enabling strict includes this option. Narrow the value before reading properties such as message:
try {
runTask();
} catch (error) {
if (error instanceof Error) {
console.error(error.message);
}
}The TypeScript 4.4 release notes document the option and demonstrate narrowing with instanceof Error.
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Further reading
For broader TypeScript guidance, O’Reilly lists a second edition of Effective TypeScript, updated for TypeScript 5 and including advice on limiting any. It is optional reading; the TypeScript Handbook links above address the specific language behavior discussed here.
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