Today we’re happy to announce the availability of the release candidate of TypeScript 3.6. This release candidate is intended to be fairly close to the full release, and will stabilize for the next few weeks leading up to our official release.<\/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 explore what’s coming in 3.6!<\/p>\n TypeScript 3.6 introduces stricter checking for iterators and generator functions. In earlier versions, users of generators had no way to differentiate whether a value was yielded or returned from a generator.<\/p>\n Additionally, generators just assumed the type of In TypeScript 3.6, the checker now knows that the correct type for The Building on that work, the new To allow differentiation between returned values and yielded values, TypeScript 3.6 converts the In short, what this means is that you’ll be able to appropriately narrow down values from iterators when dealing with them directly.<\/p>\n To correctly represent the types that can be passed in to a generator from calls to If you’d prefer to be explicit, you can also enforce the type of values that can be returned, yielded, and evaluated from npm install -g typescript@rc<\/pre>\n<\/div>\n
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Stricter Generators<\/h2>\n
function<\/span>*<\/span> foo() {\r\n if<\/span> (Math<\/span>.random<\/span>() <<\/span> 0.5<\/span>) yield<\/span> 100<\/span>;\r\n return<\/span> \"<\/span>Finished!\"<\/span><\/span>\r\n}\r\n\r\nlet<\/span> iter =<\/span> foo<\/span>();\r\nlet<\/span> curr =<\/span> iter<\/span>.next<\/span>();\r\nif<\/span> (curr<\/span>.done<\/span>) {\r\n \/\/ TypeScript 3.5 and prior thought this was a 'string | number'.<\/span>\r\n \/\/ It should know it's 'string' since 'done' was 'true'!<\/span>\r\n curr<\/span>.value<\/span>\r\n}<\/pre>\n<\/div>\nyield<\/code> was always any<\/code>.<\/p>\nfunction<\/span>*<\/span> bar() {\r\n let<\/span> x:<\/span> { hello():<\/span> void<\/span> } =<\/span> yield<\/span>;\r\n x<\/span>.hello<\/span>();\r\n}\r\n\r\nlet<\/span> iter =<\/span> bar<\/span>();\r\niter<\/span>.next<\/span>();\r\niter<\/span>.next<\/span>(123<\/span>); \/\/ oops! runtime error!<\/span><\/pre>\n<\/div>\ncurr.value<\/code> should be string<\/code> in our first example, and will correctly error on our call to next()<\/code> in our last example. This is thanks to some changes in the Iterator<\/code> and IteratorResult<\/code> type declarations to include a few new type parameters, and to a new type that TypeScript uses to represent generators called the Generator<\/code> type.<\/p>\nIterator<\/code> type now allows users to specify the yielded type, the returned type, and the type that next<\/code> can accept.<\/p>\ninterface<\/span> Iterator<\/span><T<\/span>, TReturn<\/span> =<\/span> any<\/span>, TNext<\/span> =<\/span> undefined<\/span>> {\r\n \/\/ Takes either 0 or 1 arguments - doesn't accept 'undefined'<\/span>\r\n next(...<\/span>args<\/span>:<\/span> [] |<\/span> [TNext<\/span>]):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n return?<\/span>(value<\/span>?<\/span>:<\/span> TReturn<\/span>):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n throw?<\/span>(e<\/span>?<\/span>:<\/span> any<\/span>):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n}<\/pre>\n<\/div>\nGenerator<\/code> type is an Iterator<\/code> that always has both the return<\/code> and throw<\/code> methods present, and is also iterable.<\/p>\ninterface<\/span> Generator<\/span><T<\/span> =<\/span> unknown<\/span>, TReturn<\/span> =<\/span> any<\/span>, TNext<\/span> =<\/span> unknown<\/span>>\r\n extends<\/span> Iterator<\/span><T<\/span>, TReturn<\/span>, TNext<\/span>> {\r\n next(...<\/span>args<\/span>:<\/span> [] |<\/span> [TNext<\/span>]):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n return(value<\/span>:<\/span> TReturn<\/span>):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n throw(e<\/span>:<\/span> any<\/span>):<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span>>;\r\n [Symbol<\/span>.iterator<\/span>]():<\/span> Generator<\/span><T<\/span>, TReturn<\/span>, TNext<\/span>>;\r\n}<\/pre>\n<\/div>\nIteratorResult<\/code> type to a discriminated union type:<\/p>\ntype<\/span> IteratorResult<\/span><T<\/span>, TReturn<\/span> =<\/span> any<\/span>> =<\/span> IteratorYieldResult<\/span><T<\/span>> |<\/span> IteratorReturnResult<\/span><TReturn<\/span>>;\r\n\r\ninterface<\/span> IteratorYieldResult<\/span><TYield<\/span>> {\r\n done?<\/span>:<\/span> false<\/span>;\r\n value:<\/span> TYield<\/span>;\r\n}\r\n\r\ninterface<\/span> IteratorReturnResult<\/span><TReturn<\/span>> {\r\n done:<\/span> true<\/span>;\r\n value:<\/span> TReturn<\/span>;\r\n}<\/pre>\n<\/div>\nnext()<\/code>, TypeScript 3.6 also infers certain uses of yield<\/code> within the body of a generator function.<\/p>\nfunction<\/span>*<\/span> foo() {\r\n let<\/span> x:<\/span> string<\/span> =<\/span> yield<\/span>;\r\n console<\/span>.log<\/span>(x<\/span>.toUpperCase<\/span>());\r\n}\r\n\r\nlet<\/span> x =<\/span> foo<\/span>();\r\nx<\/span>.next<\/span>(); \/\/ first call to 'next' is always ignored<\/span>\r\nx<\/span>.next<\/span>(42<\/span>); \/\/ error! 'number' is not assignable to 'string'<\/span><\/pre>\n<\/div>\nyield<\/code> expressions using an explicit return type. Below, next()<\/code> can only be called with boolean<\/code>s, and depending on the value of done<\/code>, value<\/code> is either a string<\/code> or a number<\/code>.<\/p>\n\/**<\/span>\r\n * - yields numbers<\/span>\r\n * - returns strings<\/span>\r\n * - can be passed in booleans<\/span>\r\n *\/<\/span>\r\nfunction<\/span>*<\/span> counter():<\/span> Generator<\/span><number<\/span>, string<\/span>, boolean<\/span>> {\r\n let<\/span> i =<\/span> 0<\/span>;\r\n while<\/span> (true<\/span>) {\r\n if<\/span> (yield<\/span> i<\/span>++<\/span>) {\r\n break<\/span>;\r\n }\r\n }\r\n return<\/span>