Why String.prototype.substr() seems to be deprecated?

こ雲淡風輕ζ 提交于 2019-12-01 16:46:25

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), which said:

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, but the second argument has a different meaning, as well as with slice.

In pragmatic terms, I'd be surprised if you found a mainstream JavaScript engine that didn't provide it.

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