How can you specifically target an array for comparison? I can rule out undefined and Not a Num but can't rule out str

Question

I am a little stuck here. would be grateful for a hand. cheers

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>JavaScript Foundations: Functions</title>
    <style>
      html {
        background: #FAFAFA;
        font-family: sans-serif;
      }
    </style>
  </head>
  <body>
    <h1>JavaScript Foundations</h1>
    <h2>Functions: Return Values</h2>
    <script>

      function arrayCounter(array)  {
        if(array === undefined || array === NaN || array === "aString") {
          return 0;
        }
        return array.length;
      }

    </script>
  </body>
</html>

Answer 1 · 2014-12-01T12:16:06Z

December 1, 2014 12:16pm

Hi, Gavin Murphy:

Objects and Arrays have traditionally have had quirky behavior in JavaScript before ES6. For example, the following will output "What is going on?" regardless of comparison check strictness [== w/ (type coercion) or === (with no type coercion)]

var arrayOfNumbers = [0, 1, 2, 3]; 
if ( arrayOfNumbers === [0, 1, 2, 3] ){
    console.log("equal arrays");
} else {
  console.log("What is going on?");
}

For that reason, many JS developers have relied on custom deepEqual or isEqual helper functions from libraries such as underscore that essentially check if two things are equal, considering if they're primitive type values (number, string, boolean, null, undefined, symbols) or reference type values (Objects such as functions, and so on).

As far as arrays, if the first object is an array, such functions will check if the second parameter is also an array and of the same length. From there, each item in an array in the first array is compared to the value with the same index if the first test(s) pass.

If both values are reference types, both are iterated again till equality can be truly verified, returning false immediately if that can be verified.

For what it's worth, including the quirkiness that is associated with NaN as well, the committee that creates the standards for EcmaScript6 (something that is then adhered to by the next version of JavaScript) has specified plans for a Object.is method being available to JavaScript developers that should minimize the need for third-party libraries or custom methods from scratch dealing with this

However, some transpilers or some browsers with Object.is support may still not produce a reliable result.

For example I got the wrong output again when I used the implementation available on ES6Fiddle.net, a popular resource to try out ECMAScript6 JavaScript (ES6 JavaScript) concepts:

let arrayOfNumbers = [0, 1, 2, 3]; 
if ( Object.is( arrayOfNumbers, [0, 1, 2, 3])  ){
    console.log("equal arrays");
} else {
  console.log("What is going on?");
}

Accordingly, I recommend taking advantage of the helper methods that address this problem from lodash or underscore in the meantime.

For more information on this subject, I recommend this StackOverflow post

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Gavin Murphy

Gavin Murphy

How can you specifically target an array for comparison? I can rule out undefined and Not a Num but can't rule out str

1 Answer

Kevin Lozandier

Kevin Lozandier