Today we’re excited to announce our Release Candidate of TypeScript 4.2! Between now and the stable release of TypeScript 4.2, we expect no further changes apart from critical bug fixes.<\/p>\n
To get started using the RC, you can get it through NuGet<\/a>, or use npm with the following command:<\/p>\n You can also get editor support by<\/p>\n Let’s take a look at what’s in store for TypeScript 4.2!<\/p>\n In TypeScript, tuple types are meant to model arrays with specific lengths and element types.<\/p>\n Over time, TypeScript’s tuple types have become more and more sophisticated, since they’re also used to model things like parameter lists in JavaScript. As a result, they can have optional elements and rest elements, and can even have labels for tooling and readability.<\/p>\n In TypeScript 4.2, rest elements specifically been expanded in how they can be used. In prior versions, TypeScript only allowed However, now rest elements can occur anywhere<\/em> within a tuple – with only a few restrictions.<\/p>\n The only restriction is that a rest element can be placed anywhere in a tuple, so long as it’s not followed by another optional element or rest element. In other words, only one rest element per tuple, and no optional elements after rest elements.<\/p>\nnpm install typescript@rc<\/pre>\n<\/div>\n
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Leading\/Middle Rest Elements in Tuple Types<\/h2>\n
\/\/ A tuple that stores a pair of numbers<\/span>\r\nlet<\/span> a<\/span>: [<\/span>number<\/span>,<\/span> number<\/span>]<\/span> =<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>]<\/span>;<\/span>\r\n\r\n\/\/ A tuple that stores a string, a number, and a boolean<\/span>\r\nlet<\/span> b<\/span>: [<\/span>string<\/span>,<\/span> number<\/span>,<\/span> boolean<\/span>]<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> 42<\/span>,<\/span> true<\/span>]<\/span>;<\/span><\/pre>\n<\/div>\n\/\/ A tuple that has either one or two strings.<\/span>\r\nlet<\/span> c<\/span>: [<\/span>string<\/span>,<\/span> string<\/span>?]<\/span> =<\/span> [<\/span>\"hello\"<\/span>]<\/span>;<\/span>\r\nc<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> \"world\"<\/span>]<\/span>;<\/span>\r\n\r\n\/\/ A labeled tuple that has either one or two strings.<\/span>\r\nlet<\/span> d<\/span>: [<\/span>first<\/span>: string,<\/span> second<\/span>?: string]<\/span> =<\/span> [<\/span>\"hello\"<\/span>]<\/span>;<\/span>\r\nd<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> \"world\"<\/span>]<\/span>;<\/span>\r\n\r\n\/\/ A tuple with a *rest element* - holds at least 2 strings at the front,<\/span>\r\n\/\/ and any number of booleans at the back.<\/span>\r\nlet<\/span> e<\/span>: [<\/span>string<\/span>,<\/span> string<\/span>,<\/span> ...boolean<\/span>[<\/span>]<\/span>]<\/span>;<\/span>\r\n\r\ne<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> \"world\"<\/span>]<\/span>;<\/span>\r\ne<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> \"world\"<\/span>,<\/span> false<\/span>]<\/span>;<\/span>\r\ne<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> \"world\"<\/span>,<\/span> true<\/span>,<\/span> false<\/span>,<\/span> true<\/span>]<\/span>;<\/span><\/pre>\n<\/div>\n...rest<\/code> elements at the very last position of a tuple type.<\/p>\nlet<\/span> foo<\/span>: [<\/span>...string<\/span>[<\/span>]<\/span>,<\/span> number<\/span>]<\/span>;<\/span>\r\n\r\nfoo<\/span> =<\/span> [<\/span>123<\/span>]<\/span>;<\/span>\r\nfoo<\/span> =<\/span> [<\/span>\"hello\"<\/span>,<\/span> 123<\/span>]<\/span>;<\/span>\r\nfoo<\/span> =<\/span> [<\/span>\"hello!\"<\/span>,<\/span> \"hello!\"<\/span>,<\/span> \"hello!\"<\/span>,<\/span> 123<\/span>]<\/span>;<\/span>\r\n\r\nlet<\/span> bar<\/span>: [<\/span>boolean<\/span>,<\/span> ...string<\/span>[<\/span>]<\/span>,<\/span> boolean<\/span>]<\/span>;<\/span>\r\n\r\nbar<\/span> =<\/span> [<\/span>true<\/span>,<\/span> false<\/span>]<\/span>;<\/span>\r\nbar<\/span> =<\/span> [<\/span>true<\/span>,<\/span> \"some text\"<\/span>,<\/span> false<\/span>]<\/span>;<\/span>\r\nbar<\/span> =<\/span> [<\/span>true<\/span>,<\/span> \"some\"<\/span>,<\/span> \"separated\"<\/span>,<\/span> \"text\"<\/span>,<\/span> false<\/span>]<\/span>;<\/span><\/pre>\n<\/div>\ninterface<\/span> Clown<\/span> {<\/span> \/*...*\/<\/span> }<\/span>\r\ninterface<\/span> Joker<\/span> {<\/span> \/*...*\/<\/span> }<\/span>\r\n\r\nlet<\/span> StealersWheel<\/span>: [<\/span>...Clown<\/span>[<\/span>]<\/span>,<\/span> \"me\"<\/span>,<\/span> ...Joker<\/span>[<\/span>]<\/span>]<\/span>;<\/span>\r\n\/\/ ~~~~~~~~~~ Error!<\/span>\r\n\/\/ A rest element cannot follow another rest element.<\/span>\r\n\r\nlet<\/span> StringsAndMaybeBoolean<\/span>: [<\/span>...string<\/span>[<\/span>]<\/span>