Скачать или смотреть Understanding the never Type in TypeScript: Why Does number & string Result in never?

Discover why TypeScript returns `never` when using an intersection of `number` and `string`, and understand the implications of such type declarations.
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Understanding the never Type in TypeScript

When coding in TypeScript, you might encounter the never type under certain conditions that can leave you puzzled. One such scenario arises when you declare an intersection type combining two fundamentally incompatible types. For instance, when you declare type q = number & string;, why does it return the dreaded never type? Let’s break it down in a clear and engaging manner.

The Intersection of Types

What Are Intersection Types?

In TypeScript, an intersection type allows you to combine multiple types into one. The resulting type will include all the properties of both types. For example:

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In this case, a variable of type Person would need to have both a name of type string and an age of type number.

Why number & string Results in never

When we declare the type as number & string, we are looking for a type that is both number and string at the same time. Here’s why this scenario leads to the never type:

No Overlap: The number and string types are distinct and entirely separate in TypeScript. There aren't any values that can be classified as both a number and a string simultaneously.

Understanding never: In TypeScript, the never type signifies values that can never occur. Thus, the outcome of attempting to find a commonality between number and string results in never, as no value can fit this criteria.

A Visual Analogy

To illustrate this concept, think about it like trying to find something that is both a left hand and a right hand at the same time. This is impossible! Just as there is no object that can simultaneously fulfill both criteria, there is no value that can be both a number and a string. Hence, the result is never—indicating that this situation cannot occur.

Conclusion

Understanding why type q = number & string; leads to a never type is crucial for effectively utilizing TypeScript's type system. The never type helps to enforce type safety and provides clear feedback when the code attempts to declare impossible types. The underlying principle is simple: when two types cannot coexist, TypeScript succinctly indicates this with never, helping you to design more robust applications.

By keeping these concepts in mind, you'll not only improve your coding skills but also ensure that you leverage TypeScript's features to create safer and more reliable code. If you ever find yourself dealing with seemingly contradictory types, just remember: when in doubt, consider the intersection—and keep an eye out for the elusive never.