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[Typescript] Prevent Type Widening of Object Literals with TypeScript's const Assertions
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A const
assertion is a special type assertion that uses the const
keyword instead of a specific type name. When using a const
assertion on an object literal expression, all properties will become readonly
properties and no literal types within the expression will be widened.
const ORIGIN = { x: 0, y: 0 } // works ORIGIN.x = 1
If you don't want user able to change it:
const ORIGIN: { readonly x: number, readonly y: number } = { x: 0, y: 0 } // Typescript complian // Cannot assign to 'x' because it is a read-only property. ORIGIN.x = 1
Better way:
const ORIGIN = { x: 0, y: 0 } as const // Typescript complian // Cannot assign to 'x' because it is a read-only property. ORIGIN.x = 1
But `as const` doesn't enforce in runtime. If we do want to prevent mutate the object, we can do:
const ORIGIN = Object.freeze({ x: 0, y: 0 } as const) // Typescript complian // Cannot assign to 'x' because it is a read-only property. ORIGIN.x = 1
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