javascript - Check value of variable IF it has been defined

I have an if statement, which ideally I'd like to check the value of a variable, only if the variable exists. My if statement looks something like this:

if(  variable_1 == "value_1" & 
     variable_2 == "value_2"){
   // code to execute
} 

however, sometimes variable_2 won't have been defined, and so the code crashes when that happens. Is there an elegant way to check if variable_2 exists, and if it does exist check what its value is?

Thanks is advance! Apologies if I've overlooked something basic.