[javascript] find the array index of an object with a specific key value in underscore

In underscore, I can successfully find an item with a specific key value

var tv = [{id:1},{id:2}]
var voteID = 2;
var data = _.find(tv, function(voteItem){ return voteItem.id == voteID; });
//data = { id: 2 }

but how do I find what array index that object occurred at?

If your target environment supports ES2015 (or you have a transpile step, eg with Babel), you can use the native Array.prototype.findIndex().

Given your example

const array = [ {id:1}, {id:2} ]
const desiredId = 2;
const index = array.findIndex(obj => obj.id === desiredId);

This is to help lodash users. check if your key is present by doing:

hideSelectedCompany(yourKey) {
 return _.findIndex(yourArray, n => n === yourKey) === -1;
}

Array.prototype.getIndex = function (obj) {
    for (var i = 0; i < this.length; i++) {
        if (this[i][Id] == obj.Id) {
            return i;
        }
    }
    return -1;
}

List.getIndex(obj);

If you want to stay with underscore so your predicate function can be more flexible, here are 2 ideas.

Method 1

Since the predicate for _.find receives both the value and index of an element, you can use side effect to retrieve the index, like this:

var idx;
_.find(tv, function(voteItem, voteIdx){ 
   if(voteItem.id == voteID){ idx = voteIdx; return true;}; 
});

Method 2

Looking at underscore source, this is how _.find is implemented:

_.find = _.detect = function(obj, predicate, context) {
  var result;
  any(obj, function(value, index, list) {
    if (predicate.call(context, value, index, list)) {
      result = value;
      return true;
    }
  });
  return result;
};

To make this a findIndex function, simply replace the line result = value; with result = index; This is the same idea as the first method. I included it to point out that underscore uses side effect to implement _.find as well.

you can use indexOf method from lodash

var tv = [{id:1},{id:2}]
var voteID = 2;
var data = _.find(tv, function(voteItem){ return voteItem.id == voteID; });
var index=_.indexOf(tv,data);

I got similar case but in contrary is to find the used key based on index of a given object's. I could find solution in underscore using Object.values to returns object in to an array to get the occurred index.

var tv = {id1:1,id2:2};_x000D_
var voteIndex = 1;_x000D_
console.log(_.findKey(tv, function(item) {_x000D_
  return _.indexOf(Object.values(tv), item) == voteIndex;_x000D_
}));

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js"></script>

findIndex was added in 1.8:

index = _.findIndex(tv, function(voteItem) { return voteItem.id == voteID })

See: http://underscorejs.org/#findIndex

Alternatively, this also works, if you don't mind making another temporary list:

index = _.indexOf(_.pluck(tv, 'id'), voteId);

See: http://underscorejs.org/#pluck

If you're expecting multiple matches and hence need an array to be returned, try:

_.where(Users, {age: 24})

If the property value is unique and you need the index of the match, try:

_.findWhere(Users, {_id: 10})

Keepin' it simple:

// Find the index of the first element in array
// meeting specified condition.
//
var findIndex = function(arr, cond) {
  var i, x;
  for (i in arr) {
    x = arr[i];
    if (cond(x)) return parseInt(i);
  }
};

var idIsTwo = function(x) { return x.id == 2 }
var tv = [ {id: 1}, {id: 2} ]
var i = findIndex(tv, idIsTwo) // 1

Or, for non-haters, the CoffeeScript variant:

findIndex = (arr, cond) ->
  for i, x of arr
    return parseInt(i) if cond(x)

The simplest solution is to use lodash:

1. Install lodash:

npm install --save lodash

2. Use method findIndex:

const _ = require('lodash');

findIndexByElementKeyValue = (elementKeyValue) => {
  return _.findIndex(array, arrayItem => arrayItem.keyelementKeyValue);
}

Lo-Dash, which extends Underscore, has findIndex method, that can find the index of a given instance, or by a given predicate, or according to the properties of a given object.

In your case, I would do:

var index = _.findIndex(tv, { id: voteID });

Give it a try.