Performance Implications When Comparing Types in Node.js

Like in any language that is weakly typed, you can’t avoid the fact that performing comparisons across types will cost you CPU cycles.

Consider the following code which does a .filter on an array of 5M entries, all of which are Numbers:

let arrOfNumbers = Array(5000000).fill(1);
console.time('eqeq-number')
arrOfNumbers.filter(a => a == 1)
console.timeEnd('eqeq-number')
console.time('eqeqeq-number')
arrOfNumbers.filter(a => a === 1)
console.timeEnd('eqeqeq-number')

On my Mac, they’re roughly equivalent, with a marginal difference in the performance in the eqeq and eqeqeq case:

eqeq-number: 219.409ms
eqeqeq-number: 225.197ms

I would have assumed that the eqeqeq would have been faster given there’s no possibility of data type coercion, but it’s possible the VM knew everything was a numberĀ  in the array and the test value, so, meh, about the same.

Now, for the worst case scenario, consider this following code: the same .filter, but the array is now full of 5M strings of the value “1”:

let arrOfStrings = Array(5000000).fill('1');
console.time('eqeq-string')
arrOfStrings.filter(a => a == 1)
console.timeEnd('eqeq-string')
console.time('eqeqeq-string')
arrOfStrings.filter(a => a === 1)
console.timeEnd('eqeqeq-string')

The eqeq costs about the same as the original example with the weakly typed Number to Number comparison, but now the eqeqeq is significantly faster:

eqeq-string: 258.572ms
eqeqeq-string: 72.275ms

In this case it’s clear to see that the eqeqeq case doesn’t have to do any data coercion since the types don’t match, the evaluation is automatically false without having to muck the String to a Number. If you were to continue to mess around and have the .filters compare eqeq and eqeqeq to a String ‘1’ the results again are the same as the first few tests.

Conclusion? Same the VM work if you can. This is a really obtuse example as the eqeqeq can quickly shortcut the comparison to “false” since the types don’t match, but anywhere you can save effort when working on large data sets, it’s helpful to do so, and typing is an easy win when you can take it.