javascript - lastIndexOf 的默认值是多少？

Question

string.lastIndexOf(searchValue[, fromIndex])

MDN说 fromIndex 默认值等于 string.length，但是，我真的认为它是 string.length-1

但我怎么想并不重要...我需要有人确认 fromIndex 的默认值是什么

他们是这么说的:

"It can be any integer between 0 and the length of the string. The default value is the length of the string."

Answer 1

根据 ECMAScript 5，它将是字符串的长度。

15.5.4.8 String.prototype.lastIndexOf (searchString, position)

If position is undefined, the length of the String value is assumed, so as to search all of the String.

...

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let searchStr be ToString(searchString).
4. Let numPos be ToNumber(position). (If position is undefined, this step produces the value NaN).
5. If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
6. Let len be the number of characters in S.
7. Let start min(max(pos, 0), len).
8. Let searchLen be the number of characters in searchStr.
9. Return the largest possible nonnegative integer k not larger than start such that k+ searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as the character at position j of searchStr; but if there is no such integer k, then return the value -1.