How to find index of an object by key and value in an javascript array

The Functional Approach

All the cool kids are doing functional programming (hello React users) these days so I thought I would give the functional solution. In my view it's actually a lot nicer than the imperatival for and each loops that have been proposed thus far and with ES6 syntax it is quite elegant.

Update

There's now a great way of doing this called findIndex which takes a function that return true/false based on whether the array element matches (as always, check for browser compatibility though).

var index = peoples.findIndex(function(person) {
  return person.attr1 == "john"
});

With ES6 syntax you get to write this:

var index = peoples.findIndex(p => p.attr1 == "john");

The (Old) Functional Approach

TL;DR

If you're looking for index where peoples[index].attr1 == "john" use:

var index = peoples.map(function(o) { return o.attr1; }).indexOf("john");

Explanation

Step 1

Use .map() to get an array of values given a particular key:

var values = object_array.map(function(o) { return o.your_key; });

The line above takes you from here:

var peoples = [
  { "attr1": "bob", "attr2": "pizza" },
  { "attr1": "john", "attr2": "sushi" },
  { "attr1": "larry", "attr2": "hummus" }
];

To here:

var values = [ "bob", "john", "larry" ];

Step 2

Now we just use .indexOf() to find the index of the key we want (which is, of course, also the index of the object we're looking for):

var index = values.indexOf(your_value);

Solution

We combine all of the above:

var index = peoples.map(function(o) { return o.attr1; }).indexOf("john");

Or, if you prefer ES6 syntax:

var index = peoples.map((o) => o.attr1).indexOf("john");

Demo:

var peoples = [
  { "attr1": "bob", "attr2": "pizza" },
  { "attr1": "john", "attr2": "sushi" },
  { "attr1": "larry", "attr2": "hummus" }
];

var index = peoples.map(function(o) { return o.attr1; }).indexOf("john");
console.log("index of 'john': " + index);

var index = peoples.map((o) => o.attr1).indexOf("larry");
console.log("index of 'larry': " + index);

var index = peoples.map(function(o) { return o.attr1; }).indexOf("fred");
console.log("index of 'fred': " + index);

var index = peoples.map((o) => o.attr2).indexOf("pizza");
console.log("index of 'pizza' in 'attr2': " + index);

If you want to check on the object itself without interfering with the prototype, use hasOwnProperty():

var getIndexIfObjWithOwnAttr = function(array, attr, value) {
    for(var i = 0; i < array.length; i++) {
        if(array[i].hasOwnProperty(attr) && array[i][attr] === value) {
            return i;
        }
    }
    return -1;
}

to also include prototype attributes, use:

var getIndexIfObjWithAttr = function(array, attr, value) {
    for(var i = 0; i < array.length; i++) {
        if(array[i][attr] === value) {
            return i;
        }
    }
    return -1;
}

Using jQuery .each()

var peoples = [
  { "attr1": "bob", "attr2": "pizza" },
  { "attr1": "john", "attr2": "sushi" },
  { "attr1": "larry", "attr2": "hummus" }
];

$.each(peoples, function(index, obj) {
   $.each(obj, function(attr, value) {
      console.log( attr + ' == ' + value );
   });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

Using for-loop:

var peoples = [
  { "attr1": "bob", "attr2": "pizza" },
  { "attr1": "john", "attr2": "sushi" },
  { "attr1": "larry", "attr2": "hummus" }
];

for (var i = 0; i < peoples.length; i++) {
  for (var key in peoples[i]) {
    console.log(key + ' == ' + peoples[i][key]);
  }
}

Not a direct answer to your question, though I thing it's worth mentioning it, because your question seems like fitting in the general case of "getting things by name in a key-value storage".

If you are not tight to the way "peoples" is implemented, a more JavaScript-ish way of getting the right guy might be :

var peoples = {
  "bob":  { "dinner": "pizza" },
  "john": { "dinner": "sushi" },
  "larry" { "dinner": "hummus" }
};

// If people is implemented this way, then
// you can get values from their name, like :
var theGuy = peoples["john"];

// You can event get directly to the values
var thatGuysPrefferedDinner = peoples["john"].dinner;

Hope if this is not the answer you wanted, it might help people interested in that "key/value" question.

function getIndexByAttribute(list, attr, val){
    var result = null;
    $.each(list, function(index, item){
        if(item[attr].toString() == val.toString()){
           result = index;
           return false;     // breaks the $.each() loop
        }
    });
    return result;
}