TypeScript 4.6 Improves Constructors
Written by Kay Ewbank   
Thursday, 03 March 2022

TypeScript 4.6 has been released with improvements including the ability to have code before super() in constructors, and control flow analysis for destructured discriminated unions.

TypeScript is a superset of JavaScript that adds optional static types which can be checked by the TypeScript compiler to catch common errors in your programs. TypeScript can use this information to help you avoid about mistakes like typos, missing arguments, or forgetting to check for null and undefined.

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The first improvement to TypeScript 4.6 is the ability to have code in constructors before super(). In JavaScript classes it’s mandatory to call super() before referring to 'this'. Until now, TypeScript's enforcement of this rule was rather too strict, resulting in an error if there was any code at the beginning of a constructor if its containing class had any property initializers. TypeScript 4.6 is now much more lenient in that check and permits other code to run before super(). while still ensuring that super() occurs at the top-level before any references to 'this'.

Another improvement is the addition of control flow analysis for destructured discriminated unions. TypeScript can use discriminant properties to narrow the type analysis. If an individual property is destructured into a const declaration, TypeScript checks to see if the destructured type is a discriminated union. If it is, TypeScript can now narrow the types of variables depending on checks of other variables

Other improvements include improved recursion depth checks and improvements to indexed access types meaning that TypeScript can now correctly infer to indexed access types which immediately index into a mapped object type.

There's also a new tool called the TypeScript Trace Analyzer. This gives a better view of the analysis showing when a type could be computationally expensive to create and compare against other types.

TypeScript 4.6 is available now.

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More Information

TypeScript On NuGet

TypeScript On GitHub

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TypeScript 4.1 Adds Temporal Literal Types

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