为什么不推荐 String.Prototype.subr() ？

ECMAScript 标准 给你提到:

... 这些特性不被认为是核心 ECMAScript 的一部分 程序员不应该使用或假设这些 编写新的 ECMAScript 代码时的特性和行为 实现不鼓励实现这些特性 除非该实现是网络浏览器的一部分或必须 运行 Web 浏览器遇到的相同的遗留 ECMAScript 代码。

MDN 上还有一个红色警告: Subr () MDN 文档

有人知道为什么(ECMAScript 标准是这么说的)程序员不应该使用或者假设 String.prototype.substr的存在吗？

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最佳答案

Because it's never been part of the standardized language. It wasn't in the ECMAScript 1 or 2 specs at all, and only appears in ECMAScript 3 in Section B.2 ("Additional Properties") (and subsequent editions in similar annexes through to today [ES2022 draft as of this writing]), which said:¹

Some implementations of ECMAScript have included additional properties for some of the standard native objects. This non-normative annex suggests uniform semantics for such properties without making the properties or their semantics part of this standard.

Moreover, substr is largely redundant with substring and slice, but the second argument has a different meaning,.

In pragmatic terms, I'd be surprised if you found a full mainstream JavaScript engine that didn't provide it; but I wouldn't be surprised if JavaScript engines targeted at embedded/constrained environments use didn't provide it.

¹ That wording has changed more recently to:

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

NOTE This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

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I've added this declaration as an ambient declaration in a top-level ambient.d.ts file:

interface String {
/**
* Gets a substring beginning at the specified location and having the specified length.
* (deprecation removed)
* @param from The starting position of the desired substring. The index of the first character in the string is zero.
* @param length The number of characters to include in the returned substring.
*/
substr(from: number, length?: number): string;
}

I find substr very useful. It's often a lot more concise to specify how long you want a string to be rather than the end index of the string. If substr really ever is removed from browser or Node.js JavaScript support, I suspect many of us will simply monkey-patch substr back into existence anyway.

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The main advantage of substr is that you can specify a negative start position! To do the same with substring is awful.


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