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I have an array of objects, all with the same keys. I want to check if 2 keys, in this case "a" & "b" from ALL OBJECTS, have the same value. Meaning a === a and b === b, across all objects.

var arr = [{a:"x", b:2, c:3, d:4},{a:1, b:2, c:3, d:4},{a:1, b:2, c:3, d:4},{a:1, b:2, c:3, d:4}];

if one of the key values(a or b) doesn't match the others, I want to return false. This case should return false because "a" in arr[0] = "x", while all the others equal 1;

Here's my attempt. Obviously doesnt work but what I was attempting was to have 2 loops compare 2 objects at a time. For example, the first loop will start at arr[0] and second loop will start at arr[1], compare then move onto comparing arr[3] with arr[4] and so on. Though the length of the array wont always be even.

function compareKeyVals(arr){

  for(var i = 0; i<arr.length; i+=2){
    for(var j = 1; j<arr.length; j+=2){
     for(key in arr[i]){     
        for(keyz in arr[j]){      
          if(key === "a" && keyz === "a"){
            if(arr[i][key] != arr[j][keyz]){
              return false;
            }
          }  
        }
      }    
    }
  }
}

compareKeyVals(arr);

thanks.

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3 Answers 3

Reset to default
1

Since they all have to be equal, you can just check the a and b properties of the first object and compare all others against that. Like so:

for(var i=1; i<arr.length; i++){
  if(arr[i].a !== arr[0].a || arr[i].b !== arr[0].b) {
    return false;
  }
}
return true;

Update: Changed to strict comparison, inspired by Marc's answer

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4 Comments

Note that this will return true if there are no objects in arr
True, it will also cause an error if there is only one element in the array. But in these two cases the whole check is meaningless anyway. But a real implementation should of course be able to handle the edge cases too.
Well if there is only one element, it doesn't go into the for loop, and returns true
Obviously! Thanks. Shouldn't answer questions after having whisky.
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You can really just detect a change from one iteration to the next, right?

 var arrayLength = arr.length; //cache the array length
 if (arrayLength > 0) { //only proceed if there are some elements
   var firstA = arr[0].a; //set the baseline that all others must match
   var firstB = arr[0].b;
   for(var i = 1; i<arrayLength ; i++) {
       //if the new one doesn't match the first one, break out
       if (firstA !== arr[i].a || firstB !== arr[i].b) {
           return false;
       }
   }
   return true; //return true if we complete the loop with all matches
 } else {
   return false; //assumes you want to return false if no elements in array
 }

This will break you out of the loop immediately when it finds a mismatch, so you're not wasting iterations.

(added a check for empty array per comment)

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1 Comment

Careful, if arr has no elements, this will fail.
1 
var flag = true;  
for(var i = 1; i<arr.length; i++){
if(arr[0].a != arr[i].a || arr[0].b != arr[i].b){
flag = false;
break;
 }
}
return flag;
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1 Comment

Since both if statements do the same thing, you can combine them. Also as soon as something is false, you can break and return false. Look at the other answers for examples

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