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Episode 6: undefined vs not defined in JS

Memory Allocation in JavaScript​

  • JavaScript allocates memory to variables and functions before executing any code.
  • Even before a line of code is run, memory space is reserved for variables.
  • The value of a variable that hasn't been assigned is undefined.
  • undefined acts as a placeholder or default value in memory until a variable is assigned a different value.

Difference between undefined and not defined​

  • undefined means that memory has been allocated to a variable but no value has been assigned yet.
  • not defined refers to a variable that has not been declared or allocated any memory.

undefined vs Empty​

  • undefined is not equivalent to empty or null.
  • It is a special keyword in JavaScript that takes up its own memory space.
  • undefined is a placeholder until a value is assigned to a variable.

Examples of undefined​

// Example 1
var a; // Memory is allocated for 'a', but no value is assigned yet
console.log(a); // Output: undefined

// Example 2
var x;
console.log(x); // Output: undefined

// Example 3
console.log(y); // Output: ReferenceError: y 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 boolean a = true or string a = 'hello' later on.
  • Avoid assign undefined to a variable manually, Not a good practice. Watch Live On Youtube below:
undefined vs not defined in JS Youtube Link