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How to compare two arrays in javascript?

Marco Pestrin on December 01, 2020

Some days ago I needed to compare two arrays in Javascript and I trivially tried to compare them as if they were strings const serviceList = ["s...
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joshinat0r profile image
joshinat0r • • Edited

I like to abuse JSON if I just want to know if they have the exact same entries (when I know that they arrays are fairly small since JSON.stringify isn't exactly fast)

JSON.stringify(serviceList.sort()) === JSON.stringify(botType.sort())
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stereobooster •

Why do sort though? Those are not the same arrays [1, 2], [2, 1].

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joshinat0r •

Semantics, [1, 2] and [2, 1] have the same entries but aren't the same arrays.
You're right, if you wanted to know if they have the same entries in the same order you wouldn't sort them.

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stereobooster •

JSON.stringify([undefined]) === JSON.stringify([null])

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joshinat0r •

It's just creating two JSON strings and compares them, all of JSONs limitations (not only undefined or null but also Infinite, NaN, functions, symbols and such) apply. Shouldn't have to mention it as should be fairly obvious.

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Quentin Ménoret •

This is not working:

let botType = [1,2,3]
let serviceList = [1,1,1]
botType.length === serviceList.length && serviceList.length > 0 && serviceList.every(item => botType.indexOf(item) > -1)
// true
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Every element of the second list is in the first list, but they are definitely not equals.
A proper way to do this would be:

botType.length === serviceList.length && serviceList.length && serviceList.every((element, index) => botType[index] === element)
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stereobooster profile image
stereobooster • 
x = [1,2]
y = [1,2]
x.every(item => y.indexOf(item) > 1)
// false
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Marco Pestrin •

the condition is > -1

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stereobooster • • Edited 
x = [1,2]
y = [2,1]
x.every(item => y.indexOf(item) > -1)
// true
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pretaporter profile image
Maksim •

You can make a bit improvement

botType.indexOf(item) > 1 => botType.includes(item)
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stereobooster • • Edited

This will make code work, but it is O(n^2) (the same as the original code)

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Nahin Akbar •

You can use the 'fast-deep-equal' library.
npmjs.com/package/fast-deep-equal

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Marco Pestrin •

for just 1 check is not good to use a library. A library have other functions that are included but not used