javascript - does it make sense to test whether undeclared variable was defined earlier in code? -

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i'm studying javascript , have no programming background @ all. in example think understand foo undeclared (global) variable doesn't exist when bar calculated, bar = (undefined + 1) or nan. 

var bar = foo + 1,     foo = 1;

let's toward end of long bunch of code. if foo had been defined , foo = 1 intended re-set value of foo? make sense test it? think i'm opening scope can of worms.... ("scope? can't haaaandle scope!")

i thank in advance patience learning curve! answers phrase in analogy or metaphor under belt....

you can test , declare of not declared using or operator ||

foo = foo || 1;

so if foo exists, foo foo. if foo not exist, foo 1.


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