1var colors=["red","green","blue"];
2
3if(Array.isArray(colors)){
4    //colors is an array
5}

1Array.isArray([1, 2, 3]);	// true
2Array.isArray('asdf');		// false

1if(Array.isArray(myVarToTest)) {
2	// myVatToTest is an array
3} else {
4	// myVarToTest is not an array
5}

1// If you want to test if a variable (object) is an instance of 
2// an array. You can use the Array.isArray(varName) method
3// this method takes one argument, the variable or object you want to test
4// and returns true, if the argument is an array 
5// or false, if the argument is not an array
6
7if (Array.isArray(object)) {
8	// is true
9} else {
10	// is false
11}
12
13Array.isArray([1, 2, 3, 4]); // Returns true
14Array.isArray(4); // returns false
15// this could be used for flattening an array of arrays
16

1// Check if something is an Array 
2// just like you do with "typeof"
3Array.isArray([1, 2, 3]);	// true
4Array.isArray('asdf');		// false

1var extensions = ["image/jpeg","image/png","image/gif"];          
2if(extensions.indexOf("myfiletype") === -1){
3	alert("Image must be .png, .jpg or .gif");
4}