Make sure that your variable is a string.
if (typeof unknownVar === 'string') { // unknownVar is a string }
Make Sure that you have a non-empty string (not the zero-length string(“”)
if (typeof unknownVar === 'string' && unknownVar.length > 0) { // string with characters or whitespace }
If You want to reject strings that are made up of whitespace only, in which case you can use the String.trim() method.
if (typeof unknownVar === 'string' && unknownVar.trim().length > 0) { // String that is not empty or all whitespace }
If we already know unknownVar is a string
if (unknownVariable.length > 0)
const unknownVariable = new String('test');
The type of operator will return an object instead of a String because the String primitive is wrapped in a String object.
if (typeof unknownVariable === 'string' || String.prototype.isPrototypeOf(unknownVariable)) { // String wrapped in an object. }
You have a string primitive or an object that has the same prototype as String. It returns the type name of the variable as a lowercase string.
undefined boolean number bigint string symbol function object
if (unknownVar) { /* unknownVar has been declared, is not null,is not the empty string ''*/ }
It simply verifies that your value can be treated as a string.
This works because null values, undefined values, and empty strings (”) are all falsy in JavaScript. If you evaluate any of them in a conditional expression, they are treated as false.