C# String.IsNullOrEmpty Javascript equivalent

if(!theString) { alert("the string is null or empty");}

 

扩展

typeof null          // "object" (not "null" for legacy reasons)typeof undefined     // "undefined" null === undefined // false null == undefined // true null === null // true null == null // true !null // true isNaN(1 + null) // false isNaN(1 + undefined) // true