You can just check if the variable has a truthy value or not. That means if( value ) { } will evaluate to true if value is not: null undefined NaN empty string (“”) 0 false The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section. … Read more
If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example: // global scope var theFu; // theFu has been declared, but its value is undefined typeof theFu; // “undefined” But this may not be … Read more
The usual way to check if the value of a property is the special value undefined, is: if(o.myProperty === undefined) { alert(“myProperty value is the special value `undefined`”); } To check if an object does not actually have such a property, and will therefore return undefined by default when you try to access it: if(!o.hasOwnProperty(‘myProperty’)) … Read more