check if array is correct code example

Example 1: javascript check if in array

var extensions = ["image/jpeg","image/png","image/gif"];          
if(extensions.indexOf("myfiletype") === -1){
	alert("Image must be .png, .jpg or .gif");
}

Example 2: how to recognize an array in javascript

const array = [1, 2, 3];

Array.isArray(array);  // => true
Array.isArray(object); // => false
Array.isArray(string); // => false
Array.isArray(empty);  // => false

// other ways

({}).toString.call(array);   // => '[object Array]'
({}).toString.call(object);  // => '[object Object]'
({}).toString.call(string);  // => '[object String]'
({}).toString.call(empty);   // => '[object Null]'