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69 lines
1.3 KiB
Markdown
69 lines
1.3 KiB
Markdown
---
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date: 2020-10-05
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id: c0bc64a7-f2dc-4e2e-afb9-5dbc63eb8287
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title: Readonly
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---
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# Introduction
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[TypeScript's](20200929161126-typescript) type system allows you to mark
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individual properties on [interfaces](20200929162220-interfaces),
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[types](20200929163825-typescript_type_alias) and
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[class](20201009104411-typescript_classes) properties as `readonly`.
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# Examples
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## interface
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``` typescript
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function foo(config: {
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readonly bar: number,
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readonly bas: number}) {// ..
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}
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let config = { bar: 123, bas: 123 };
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foo(config);
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// You can be sure that `config` isn't changed 🌹
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```
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## type
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``` typescript
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type Foo = {
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readonly bar: number;
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readonly bas: number;
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}
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// Initialization is okay
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let foo: Foo = { bar: 123, bas: 456 };
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// Mutation is not
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foo.bar = 456; // Error: Left-hand side of assignment expression cannot be a constant or a read-only property
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```
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## class property
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``` typescript
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class Foo {
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readonly bar = 1; // OK
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readonly baz: string;
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constructor() {
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this.baz = "hello"; // OK
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}
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}
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```
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``` typescript
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type Foo = {
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bar: number;
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bas: number;
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}
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type FooReadonly = Readonly<Foo>;
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let foo: Foo = {bar: 123, bas: 456};
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let fooReadonly: FooReadonly = {bar: 123, bas: 456};
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foo.bar = 456; // Okay
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fooReadonly.bar = 456; // ERROR: bar is readonly
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```
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