Issue Splice method

The values in that hidden is d1,d2,d3,d4
var globalFileTypeId = document.getElementById('hdnFileDescription').value.split(',');
alert(globalFileTypeId[0]);         //   d1
alert(globalFileTypeId[1]);         //   d2
alert(globalFileTypeId[2]);         //   d3
alert(globalFileTypeId[3]);         //   d4
Now i remove the 1st index value from the array,
globalFileTypeId.splice(globalFileTypeId.indexOf(1), 1);
That mean i am trying to remove d2 but the d4 is removed.
alert(globalFileTypeId);            Now the value is d1,d2,d3 

 
Whyyyyy?
 

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