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Use a string literal union when you only need TypeScript to restrict a value to a known set of strings. Use an enum when named members and a runtime object are useful. If you want both runtime constants and a derived union type in ordinary JavaScript object syntax, consider an as const object.
How enums and string literal unions differ
Both can help prevent invalid values in TypeScript, but they do not provide the same runtime features. A string literal union is a type-level list of permitted strings. A regular enum also creates a JavaScript object that you can access and pass around at runtime.
| What you need | Enum | String literal union |
|---|---|---|
| Restrict values during type checking | Yes | Yes |
| Runtime object with named members | Yes, for a regular enum | No; the type alias alone is erased |
| Write a value | Status.Ready |
"ready" |
| Numeric value-to-name reverse lookup | Available for numeric enums | No |
| Fits plain JavaScript string syntax | Uses TypeScript-specific enum syntax | Yes |
The distinction matters when code needs a runtime value source—for example, to pass members to another function or inspect them. A union alias constrains values at compile time, but does not create an object containing the permitted strings. See the TypeScript Enums Handbook and literal types documentation.
What a regular enum creates at runtime
Regular enums are emitted as JavaScript objects. A string enum gives you named references such as Status.Ready, whose runtime value is a string. String enum values are readable when serialized, but string enum members do not have numeric-enum-style reverse mappings.
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Numeric enums behave differently: TypeScript emits mappings both from member names to numeric values and from numeric values back to member names. That can help with introspection, but it also means a numeric enum’s runtime object contains more than a simple list of values. For string-valued cases, do not expect numeric auto-increment or reverse lookup behavior. The Handbook describes these runtime behaviors in its enum reference.
How string literal unions help with checking and narrowing
A string literal union lists the exact strings a value may have:
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type Status = "ready" | "loading" | "failed";TypeScript can flag misspellings or other strings outside the set. Literal values can also narrow control flow. For example, checking whether a status is "ready" lets TypeScript treat that branch as the ready case. String literals are also commonly used as discriminants in unions of object types, so a check on a property can narrow the object to the matching variant. See the TypeScript documentation on narrowing.
The union itself does not provide named properties such as Status.Ready or a list to iterate over. Its constituents are type information, not a runtime collection.
Choose the form that fits your code
Choose a string literal union for a type-only set
Use a union when values are ordinary strings and the main requirement is to catch unsupported values during type checking. This keeps the value representation aligned with plain JavaScript strings and avoids creating an enum object just to express an allowed set.
Choose an enum when named runtime members help
Use a regular enum when references such as Status.Ready improve clarity and your code needs the enum object at runtime. A string enum keeps runtime values readable strings; a numeric enum additionally has reverse mappings.
Choose an as const object when you want both
An object declared with as const can provide runtime constants using ordinary object syntax, while its values can be used to derive a union type. This is useful when you need a runtime source of values but prefer not to use enum syntax. The trade-off is that deriving the type requires a type-extraction expression. The TypeScript Handbook discusses this pattern alongside alternatives to enums.
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A const enum is a separate option from both a regular enum and a string literal union. Its members are inlined at use sites and the enum is erased rather than emitted as a regular runtime object. That can look convenient, but ambient const enums have documented risks when a library and its downstream project compile against different versions, and they can conflict with isolatedModules. Review the TypeScript enum documentation before choosing this form for library code.
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