JavaScriptWeb DevelopmentTutorialProgramming Concepts
Undefined vs Not defined
Aman Suryavanshi1 min read
- In first phase (memory allocation) JS assigns each variable a placeholder called undefined.
- undefined is when memory is allocated for the variable, but no value is assigned yet.
Example 1
console.log(x); // undefined
var x = 25;
console.log(x); // 25
console.log(a); // Uncaught ReferenceError: a is not defined- If an object or variable is not even declared or found in memory allocation phase, and tried to access it then it is Not defined. Not Defined !== Undefined
- When variable is declared but not assigned with any value , then its current value is undefined. But when the variable itself is not declared but called in code, then it 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 Booleana = trueor stringa = 'hello'later on. - Never assign undefined to a variable manually. Let it happen on it's own accord.
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