how to know the value of array in javascript is string 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: see if array contains array javascript

const found = arr1.some(r=> arr2.indexOf(r) >= 0)