Checking if a variable exists in javascript

A variable is declared if accessing the variable name will not produce a ReferenceError. The expression typeof variableName !== 'undefined' will be false in only one of two cases:

• the variable is not declared (i.e., there is no var variableName in scope), or
• the variable is declared and its value is undefined (i.e., the variable’s value is not defined)

Otherwise, the comparison evaluates to true.

If you really want to test if a variable is declared or not, you’ll need to catch any ReferenceError produced by attempts to reference it:

var barIsDeclared = true; 
try{ bar; }
catch(e) {
    if(e.name == "ReferenceError") {
        barIsDeclared = false;
    }
}

If you merely want to test if a declared variable’s value is neither undefined nor null, you can simply test for it:

if (variableName !== undefined && variableName !== null) { ... }

Or equivalently, with a non-strict equality check against null:

if (variableName != null) { ... }

Both your second example and your right-hand expression in the && operation tests if the value is “falsey”, i.e., if it coerces to false in a boolean context. Such values include null, false, 0, and the empty string, not all of which you may want to discard.