Get the last index of an element in a Json array

Question

I have an array

var arr = [{name:"Steve",toppedIn:"Biology"},
           {name:"Carol",toppedIn:"Maths"},
           {name:"Steve",toppedIn:"Chemistry"}];

I want to get the last index of a name in the arr. findIndex() give the first one, how do I use lastIndex in this case?

@HenslerSoftware, i think he is looking for the last index of element satisfying a condition, but it is not clear from OP's question. — Vitaliy Terziev, Jun 08 '19 at 20:38
the findIndex function if used without argument returns an error. What are you talking about? — Mister Jojo, Jun 08 '19 at 20:40
i just gave the function name in the question not the implementation .In findIndex case i pass a function that has an item and the index and return the index when the array element matches with the element i want @MisterJojo — aeshna Kashyap, Jun 08 '19 at 20:43
I hope you will read my answer, it seems to me the fastest and most useful of all the ones I saw here — Mister Jojo, Jun 08 '19 at 21:44

Jack Bashford · Accepted Answer · 2019-06-08T20:44:27.557

1

Reverse the array first, then check whether the value was found or not, then subtract the reversed index from the array's last index to get the correct answer.

var arr = [{name:"Steve",toppedIn:"Biology"},{name:"Carol",toppedIn:"Maths"},{name:"Steve",toppedIn:"Chemistry"}];

const lastIndexOfName = name => {
  let index = [...arr].reverse().findIndex(person => person.name == name);
  console.log(index);
  console.log(arr.length);
  return index >= 0 ? arr.length - 1 - index : index;
}

console.log(lastIndexOfName("Steve"));

edited Jun 08 '19 at 20:44

answered Jun 08 '19 at 20:39

Jack Bashford

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Where did you reverse the array? – Sebastian Speitel Jun 08 '19 at 20:44
@SebastianSpeitel, hah was about to ask him the same thing then the answer was updated w/o me noticing.. Jack it almost seems that you did this recently.. :) – Vitaliy Terziev Jun 08 '19 at 20:49
@VitaliyTerziev Did what recently - updated my answer? – Jack Bashford Jun 08 '19 at 20:50
You posted your solution few minutes after OP's qeustion so I thought that you had this one ready just stting somewhere. :) – Vitaliy Terziev Jun 08 '19 at 20:54
1

Nope - just knew how to do it. – Jack Bashford Jun 08 '19 at 20:57

score 0 · Answer 2 · answered Jun 08 '19 at 20:34

0

In JS you must use

array[array.length - 1]

to access the last element

answered Jun 08 '19 at 20:34

Abhicoding

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I don't think the OP is looking for the last element in array but rather last element satisfying a condition. – Vitaliy Terziev Jun 08 '19 at 20:38
I sympathize, the question is itself an enigma to solve before understanding what it asks – Mister Jojo Jun 08 '19 at 21:34

score 0 · Answer 3 · answered Jun 08 '19 at 20:57

0

lastIndex = (arr.length-1) - arr.reverse().findIndex(x=>x.name==="Steve");
arr.reverse();

This one also works but i dont know which one is more performant.

answered Jun 08 '19 at 20:57

İbrahim Alkan

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Mister Jojo · Answer 4 · 2019-06-08T21:46:28.027

I made a simplified wrapper for this question :

var arr = [ {name:"Steve",toppedIn:"Biology"  }   // 0    Steve
          , {name:"Carol",toppedIn:"Maths"    }   // 1
          , {name:"Carol",toppedIn:"Maths"    }   // 2
          , {name:"Steve",toppedIn:"Chemistry"}   // 3    Steve
          , {name:"Carol",toppedIn:"Maths"    }   // 4
          , {name:"Carol",toppedIn:"Maths"    }   // 5
          , {name:"Steve",toppedIn:"Chemistry"}   // 6  <-- last Steve
          , {name:"Carol",toppedIn:"Maths"    }   // 7
          , {name:"Carol",toppedIn:"Maths"    }   // 8
          ];



Array.prototype.FindLastIndex = function( CallFunction ) {
  var r = -1, t = Object(this);
  for (let i=t.length;i-->0;) {
    if (CallFunction(t[i])) {
      r=i;
      break
    }
  }
  return r
};


console.log ('last Steve is: index= ',  arr.FindLastIndex(a=>a.name=='Steve')  )  // 6

Get the last index of an element in a Json array

4 Answers4