What is the best way to check if a parameter is an array?

Question

The challenge is:

"Around line 17, create a function named 'arrayCounter' that takes in a parameter which is an array. The function must return the length of an array passed in or 0 if a 'string', 'number' or 'undefined' value is passed in."

I came up with this code which passed the challenge, but I'm not sure if it is the best way to check if a parameter is an array:

      function arrayCounter(ArrayParam) {
        if ((typeof ArrayParam === 'string') || 
            (typeof ArrayParam === 'number') ||
            (typeof ArrayParam === 'undefined')){
            return 0; 
        } else {
            return ArrayParam.length; 
        }
      };

The code trys to return the length of any input that isn't a string, number, or undefined. For example, if a function or non-array object were used as the parameter, the arrayCounter function would try to return the length even though the parameter isn't an array.

Answer 1 · 2014-03-16T18:46:21Z

March 16, 2014 6:46pm

You're right that the code for the challenge won't correctly test for an Array. You're better off using JavaScript's instanceof operator. It can tell you if a variable is an array or not like this:

function arrayCounter (array) {
    if (array instanceof Array) {
        return array.length;
    } else {
        return 0;
    }
}
var myArray = [1,2,3];
console.log(arrayCounter(myArray)); // prints 3 to the console
var myString = 'test';
console.log(arrayCounter(myString)); // prints 0 to the console
var myObj = { name: 'object' };
console.log(arrayCounter(myObj)); // prints 0 to the console

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Adrian Melendez

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What is the best way to check if a parameter is an array?

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Dave McFarland

Dave McFarland