- In first phase (mem alloc) JS assigns each variable to a placeholder called undefined
- undefined is when memory is allocated for the variable, but no value is assigned yet.
- If an object/variable is not even declared/found in mem alloc phase, and tried to access it then it is Not defined
When variable is declared but not assigned value, its current value is undefined. But when the variable itself is not declared but called in code, then it is not defined.
console.log(x);
var x = 25;
console.log(x);
console.log(a);
undefined
25
Uncaught ReferenceError: a is not defined
- JS is a loosely-typed / weakly-typed language. It doesn't attach variables to any datatype. We can say var a = 5, and then change the value to bool (a = true) or string (a = 'hello') later on.
- Never assign undefined to a variable manually. Let it happen on it's own accord.