Question

in javascript,Array instance has two methods,

[].indexOf(searchvalue [,start])

and

[].lastIndexOf(searchvalue [,start])

is behaves strange if the “start” param is undefined:

[1,2,3].lastIndexOf(2) // 1
[1,2,3].lastIndexOf(2,undefined) // -1
[1,2,3].indexOf(2,undefined) // 1

this happens in chrome and firefox,so what’s the theory of the indexOf and lastIndexOf treat “undefined” differently

Answer 1

array.lastIndexOf(searchElement[, fromIndex])

fromIndex The index at which to start searching backwards. Defaults to
the array’s length, i.e. the whole array will be searched. If the
index is greater than or equal to the length of the array, the whole
array will be searched. If negative, it is taken as the offset from
the end of the array. Note that even when the index is negative, the
array is still searched from back to front. If the calculated index is
less than 0, -1 is returned, i.e. the array will not be searched.

[1,2,3].lastIndexOf(2,undefined) is same as [1,2,3].lastIndexOf(2, 0), so only the first element will be searched.

[1,2,3].lastIndexOf(2, 0) will return -1.

[1,2,3].lastIndexOf(1, 0) will return 0.