What is Temporal Dead Zone in JavaScript?
Before starting this blog, I would recommend to read blog on Hoisting in JavaScript.
The Temporal Dead Zone (TDZ) is a specific time between whenever we declare any variable using let keyword and initializing declared variable a value, the time span between these two events known to be as temporal dead zone, whenever we try to use variable in between temporal dead zone state, we get Reference Error because it will be always unreachable for the particular time being till it gets initialized.
console.log(num);
let num = 10;
In memory creation phase of the num will be hoisted inside the special memory area of Javascript engine, and in the second phase or code execution phase if engine try to execute above code value of num will be unreachable because the initialization happens in second line. so the line number 1st will give us an Reference Error.
Why TDZ not with const and var?
Because of hoisting we can access any variable created using var even before it gets declared, and we get an special value as undefined because it attached to global object.
It doesn't mean variable created with const and let are not hoisted but in the case of const the initialization must be in the same line while declaring the variable.