How to get index value of same array value in javascript

Question

Now i have an array var a = [4,7,4] in this array value 4 is same values and how can i get the index of same value.

I got some code in StackOverflow but its checking only num 2 value, i need to check every element and return index value .

JS:

var dataset = [2,2,4,2,6,4,7,8];
var results = [];
for ( i=0; i < dataset.length; i++ ){
    if ( dataset[i] == 2 ){
        results.push( i );
    }
}

return results;

JSFIDDLE

http://jsfiddle.net/friendz/42y08384/15/

Answer 1

Answer

var dataset = [2,2,4,2,6,4,7,8];
var results = [];
for ( i=0; i < dataset.length; i++ ){
    for(j=i+1;j<dataset.length;j++){
    if ( dataset[i] == dataset[j] ){
        results.push( j );
        break;
    } 
    }
}

console.log(results);

The logic behind this code is check every value with other values of array so that duplicate values' index can be found.

Answer 2

Not sur to understand what you want. The code bellow give you all index per value in the dataset.

Output:

Code

 var dataset = [2,2,4,2,6,4,7,8]; var results = {}; for ( i=0; i < dataset.length; i++ ) { if (typeof results[dataset[i]] === 'undefined') { results[dataset[i]] = []; } results[dataset[i]].push(i); } console.log(results);

Answer 3

Use reduce to build a lookup:

const out = dataset.reduce((p, c, i) => {

  // if the current value doesn't exist as a
  // key in the object, add it and assign it an
  // empty array
  p[c] = (p[c] || []);

  // push the index of the current element to its
  // associated key array
  p[c].push(i);
  return p;
}, {});

OUTPUT

{
  "2": [0, 1, 3],
  "4": [2, 5],
  "6": [4],
  "7": [6],
  "8": [7]
}

DEMO

Answer 4

Copy this code and see on console.log

 console.log(Array.from({length: 50}, (v,i) => i));

Answer 5

Sorry, here you can see: http://jsfiddle.net/42y08384/18/

var dataset = [2,2,4,2,6,4,7,8];
var results = {};
dataset.forEach(function(item, key) {
    if(!results[item]) {
    results[item] = [];
  }


  console.log(key)  
  results[item].push(key);
});

//results is an object where the key is the value from the dataset and the array within is the indexes where you can find them
for(key in results) {
    console.log('Value ' + key + ' can be found in position '+ results[key].join(', ') )
}