typescript 2.4 3.9 区别

TypeScript 2.4 vs TypeScript 3.9

TypeScript is an open-source programming language developed by Microsoft. It is a strict superset of JavaScript that adds optional static typing to the language. TypeScript provides developers with tools to write more maintainable and scalable code, as well as to catch errors during development rather than at runtime.

In this article, we will explore the differences between TypeScript 2.4 and TypeScript 3.9, two major releases of the language. We will cover some of the new features and improvements that have been introduced in TypeScript 3.9.

Nullish Coalescing Operator

One of the new features introduced in TypeScript 3.9 is the Nullish Coalescing Operator (??). It is used to provide a default value when a variable is null or undefined. This is particularly useful when you want to assign a default value to a variable but want to exclude 0 or an empty string as potential default values.

Here's an example:

const myVariable = null;
const defaultValue = 'Default Value';

const result = myVariable ?? defaultValue;

console.log(result); // Output: 'Default Value'

In the above example, the result variable is assigned the value of defaultValue because myVariable is null.

Assertion Functions

TypeScript 3.9 introduces a new type inference improvement for assertion functions. Assertion functions are functions that assert that a value has a specific type. In TypeScript 2.4, assertion functions were not fully recognized by the type checker, leading to some type inference issues.

Here's an example:

function isString(value: unknown): value is string {
  return typeof value === 'string';
}

function processString(value: unknown) {
  if (isString(value)) {
    // TypeScript 2.4: Error - Object is possibly 'null' or 'undefined'.
    // TypeScript 3.9: No error - value is correctly recognized as string.
    console.log(value.toUpperCase());
  }
}

In TypeScript 3.9, the type checker correctly recognizes that when isString(value) returns true, the value variable will have the type string. This improvement eliminates potential errors that could occur in TypeScript 2.4.

Labeled Tuple Elements

TypeScript 3.9 introduces a new syntax for labeling tuple elements. Tuple types are used to represent an array with a fixed number of elements, where each element may have a different type. With labeled tuple elements, developers can assign labels to specific elements in the tuple type.

Here's an example:

type Point = [x: number, y: number];

function printPoint(point: Point) {
  const [x, y] = point;
  console.log(`x: ${x}, y: ${y}`);
}

const myPoint: Point = [10, 20];
printPoint(myPoint); // Output: x: 10, y: 20

In the above example, the Point type is defined as a tuple with labeled elements x and y. The printPoint function then extracts the elements of the tuple and prints them with their corresponding labels.

Conclusion

TypeScript 3.9 introduced several new features and improvements over TypeScript 2.4. The Nullish Coalescing Operator provides a concise way to assign default values to variables that are null or undefined. The improvement in type inference for assertion functions helps catch potential errors and improves type safety. Labeled tuple elements allow developers to assign labels to specific elements in tuple types, enhancing code readability.

These are just a few of the differences between TypeScript 2.4 and TypeScript 3.9. TypeScript continues to evolve with each release, providing developers with new tools and features to write better code.

Remember to always stay up to date with the latest version of TypeScript to take advantage of these improvements and ensure the best developer experience.