explain that how value should store in javascript with outusing datatype "var" or "let" and how execution stack works here [duplicate]

Question

here i didn't understand what happen when i use var before variable in function it give me different out put and with out using var also i got a different output

here is a code that you can easily figure out whats going on

function value() {
  var m = 8; //here i am using var as a datatype  
  console.log(m)
}
m = 7
console.log(m);
value();
console.log(m);

and also when i removing var or not using any data type from value function then i got different out put here is a code

function value() {
  m = 8; //here i am not using var as a datatype  
  console.log(m)
}
m = 7
console.log(m);
value();
console.log(m);

can any one can tell me whats going on thanks for your time

Answer 1

Take a look at w3school's JavaScript Scope.

Below is example code of the difference of global variable and local variable.

function value() {
    // local variable in value()
    var m = 8;
    console.log("in value() : " + m)
}
function value2() {
    // set global variable as 9
    m = 9;
    console.log("in value2() : " + m)
}
// global variable
m = 7
console.log("before value() : " + m);
value();
console.log("after value() : " + m);
value2();
console.log("after value2() : " + m);

Below is another case to show the difference of scope:

m = 7;

function v(){
  var m = 6;
  v2();
  function v2(){
    console.log("in v2 : " + m);
    v3();
  }
}
function v3(){
  console.log("in v3 : " + m);
}
v();

Answer 2

Function looks for the m in the functional scope, if it does not find it there, it searches in the higher scope which over here is global scope.

In the first example, function creates a variable m in the function scope and any update will be limited to the variable in the function. So, basically in this example there are 2 variables, with one m belongs to the global scope and one m belongs to the function scope. See the interpretation of code below.

var m;
function value() {
  var m; // creates a variable m in the function scope
  m = 8;
  console.log(m)
}
m = 7
console.log(m);
value();
console.log(m);

In the second example, function tries to access a variable m in the function however, does not find there, hence, searches in the higher scope (global scope) and find one. So, basically in this example there is only 1 variable of m belongs to the global scope. See the interpretation of code below.

var m;
function value() {
  m = 8; // no function scope, variable, updates global scope
  console.log(m)
}
m = 7
console.log(m);
value();
console.log(m);