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

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

When you remove var you are accessing a global scope variable, doesn't matter that you access it inside a function. — Facundo Larrosa, Jun 15 '18 at 05:04

Terry Wei · Answer 1 · 2018-06-15T05:11:35.007

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();

Nikhil Aggarwal · Answer 2 · 2018-06-15T05:14:59.293

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);

score 3 · Answer 3 · answered Jun 15 '18 at 05:16

An identifier (m for example) lookup (getting its value) basically just checks the current scope if the variable is existing there, if not it continues to the parent scope. So inside of your function

function value() {
  m = 5;
 }

It will do a lookup like this:

function "value" scope -> execution scope -> global scope

So it finally looks it up in global scope. As there is no m variable anywhere, it will declare the variable there, so m becomes part of the global scope. Thats bad cause every script on the page can now access and modify it. If you append a var (or even better a let) it will declare the variable in the current scope (in ms case the value function scope), and when it looks up it directly finds it there. Therefore if you use another m as an identifier in the global scope, they will refer to two different variables.

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

3 Answers3