Whether a variable is undefined
37735
How can I check for "undefined" in JavaScript?
114378
How to check if the string is empty?
38133
JavaScript variables declare outside or inside loop?
99292
Why aren't variables declared in "try" in scope in "catch" or "finally"?
88299
What is the purpose of passing-in undefined?
116784
Default variable value
71634
Can a variable be defined only in the scope of an if-statement, similar to how it's often done for for-loops?
137660
Avoid running the script if a variable is not defined
151869
What's the scope of a variable initialized in an if statement?
144673
Why does this JavaScript code print "undefined" on the console?
29612
How to find the name of a variable that was passed to a function?
128709
Why is there a `null` value in JavaScript?
32989
Is there a symbol to mean 'this is undefined'?
114179
If $Var(X)=0$ then is $X$ a constant?
74517
Is using 'var' to declare variables optional?
157279
How do you check if a variable is an array in JavaScript?
11651
What does curly brackets in the `var { ... } = ...` statements do?