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javascriptlastindexof

lastIndexOf Confusion

I really dont understand how lastIndexOf works. I could not get the usage of second optional parameter.

string.lastIndexOf(searchvalue,start)

searchvalue -> Required. The string to search for

start -> Optional. The position where to start the search. If omitted, the default value is the length of the string

var test = "mississippi";

test.lastIndexOf("ss",1) // return -1
test.lastIndexOf("ss",2) // returns 2
test.lastIndexOf("ss",5) // returns 5

Could anyone tell me the idea step by step ? Why first one returns -1 and second one returns 2 for example ?

TIA

Solution

  • Its because thats the starting index. -1 means not found.

    m 0
i 1
s 2
s 3
i 4
s 5
s 6
i 7
p 8
p 9
i 10

    So starting at 1 and I dont see a match. But with 2, I see s then s at 3.

    MDN explains it well.