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Ilya Liverts
9,677 PointsHow can I check if variable is array?
My code does not pass: ''' <!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(a)
{
if(typeof a == 'string')
{
return 0;
}
if(typeof a == 'undefined')
{
return 0;
}
if(typeof a == 'number')
{
return 0;
}
if(typeof a == 'Array')
{
return a.lenght;
}
}
</script>
</body> </html> '''
2 Answers
Chris Malcolm
2,909 PointsYou misspelled length ,perhaps that has to do with error
Sean T. Unwin
28,690 Points if(typeof a == 'Array')
{
return a.lenght; /* There is a typo in length */
}
An Array is a subset of an Object so typeof will not work.
Substitute typeof for instanceof will correct the issue. There is a slight syntax change, however. See the following.
if(a instanceof Array)
{
return a.length;
}
edit: As mentioned below by Dino Paškvan, Array.isArray() is available for modern browsers and in certain use-cases would be more robust.
/**
* You still need the error checks included
* i.e. not string, number, or undefined
* and to also return 0 if so for the code challenge to pass
**/
if (Array.isArray(a)) { return a.length; }
Dino Paškvan
Courses Plus Student 44,108 PointsIt's important to note that using the instanceof operator is not a completely safe method of checking for arrays (in multi-frame DOM environments).
That's why Array.isArray() was introduced in ES5 and it can be easily polyfilled with Object.prototype.toString.call() for earlier environments.
Sean T. Unwin
28,690 PointsIndeed, Dino Paškvan.
I was going to include Array.isArray() but opted against because of some older browser compatibility. I will note, though, that this way is the best way, in my opinion, as long as the target audience's browser of the webpage is compatible. I did not want to include Object.prototype.toString.call() because I did not want to make a potentially already confusing topic to be lost on the reader.
For the purposes of the question, and more importantly, the task at hand - the code challenge - I opted for what I thought would help the most without conflating the topic.
I thank you for your response though, because a potential JavaScript developer should be aware of the intricacies of Arrays as well as Objects.
I will update my answer to include Array.isArray() as going forward there should be less use for the polyfill as up-to-date browsers will make use of the ES5 spec.