How to detect array equality in JavaScript?

Question

There are two arrays in JavaScript, they are both in the following format:

[{'drink':['alcohol', 'soft', 'hot']}, {'fruit':['apple', 'pear']}];

I need to detect if the two arrays are equal or not. they are considered equal if they contain the same elements in a different order. How can I make that?

Solution 1

1. Check the length of both arrays
2. Loop through the first array, compare each variable to the second array.

If 1 and 2 are both the same, your array is equal.

Function to compare objects/arrays:

Looping through true arrays can be achieved through for(var i=0; i<array.length; i++).
Walking through the properties of such an object can be done by for(var i in object).

function recursiveCompare(obj, reference){
    if(obj === reference) return true;
    if(obj.constructor !== reference.constructor) return false;
    if(obj instanceof Array){
         if(obj.length !== reference.length) return false;
         for(var i=0, len=obj.length; i<len; i++){
             if(typeof obj[i] == "object" && typeof reference[j] == "object"){
                 if(!recursiveCompare(obj[i], reference[i])) return false;
             }
             else if(obj[i] !== reference[i]) return false;
         }
    }
    else {
        var objListCounter = 0;
        var refListCounter = 0;
        for(var i in obj){
            objListCounter++;
            if(typeof obj[i] == "object" && typeof reference[i] == "object"){
                if(!recursiveCompare(obj[i], reference[i])) return false;
            }
            else if(obj[i] !== reference[i]) return false;
        }
        for(var i in reference) refListCounter++;
        if(objListCounter !== refListCounter) return false;
    }
    return true; //Every object and array is equal
}

If you don't understand the function, feel free to request an explanation at the comments.

Solution 2

With Javascript, you can't check if arrays are equals, but you can compare them like this:

var arr1 = ['alcohol', 'soft', 'hot'],
    arr2 = ['apple', 'pear'],
    arr3 = ['soft', 'hot', 'alcohol'];

function isSame(a1, a2){
    return !(a1.sort() > a2.sort() || a1.sort() < a2.sort());
}

console.log( isSame(arr1, arr2) ); //false
console.log( isSame(arr1, arr3) ); //true

The sort put all elements in the same order, and if both < and > comparisons are false it means both are the same.

Comments

[Atticus]

This will work for you, just keep in mind order does matter here.

[Narendra Yadala]

Yes. I was just updating the answer :)

[Atticus]

All good :) Wasn't trying to rant on your response, it's a clever way to do it. Just wanted to make sure the OP knew this since he didn't specify

[Rob W]

@Sparky Good point. I've reversed the order of the notes. I've also updated my answer, and included a comparison function.

[Sparky]

The benefits of being old and programming when languages were a lot slower...<wink>

[Mic]

Do you know you can compare arrays in Javascript? And know if they are bigger or smaller.

[Rob W]

@Mic What do you mean? Have you actually read the question + answer?

[Mic]

@Rob... I think so, did you check my answer below? Something wrong in it?

[Rob W]

Ah I see. Your Array compare function also returns true for isSame([1,2], [2,1]). The sequence of the elements is often important. Also, I've just benchmarked our functions, and my function is three times faster: jsfiddle.net/Kv383/1.

[ArchNoob]

It gives me "compareArrays" is not a function.

[Zack]

This isn't great because compareArrays is not a defined built-in function in Javascript and it's not great practice to much around with the prototype of something as fundamental as the Array class.