Keyword globalThis for undeclared var

Undeclared var variable inside a function

globalThis keyword - Javascript

If a javascript variable is used inside a function without prior declaration, still the variable will be defined and allocated as a var with global scope.

Sample Scenario - var usage

Use var in a function/non-global scope without declaration
In the sample program
- oopsglobal variable is used without declaration
- It is declared in the global scope by the nodeJS and browser interpreter during the run time.
- Name the javascript file as .cjs as CommonJS

CommonJS in NodeJS backend

Sample Code - NodeJS CommonJS javascript code

script.cjs
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                let letName = `John Doe`;
var varAge = 12;

function testoops() {
    // no declaration var means global var
    oopsglobal = 10;
}

function testlocal() {
    let local = oopsglobal;
    console.log("The Oopsglobal inside function: ", local);             // Result 10 - accessed within function undeclared
    return local;
}

testoops();
value = testlocal();

console.log("The Oops global using returned value is: ", value);        //Result 10 - return from local function
console.log("The Oops global using Global value is: ", oopsglobal);     //Result: 10 - Global usage oops global

console.log("This Value: ", this);                                      //Same as Global, Output: {} for nodejs
console.log("Global Value: ", global);                                  //Same as Global
console.log("GlobalThis Value: ", globalThis);                          //Same as Global
// console.log(window)                                                  //Same as Global,in browser (non NodeJS)

console.log("letName: ",globalThis.letName);                            //Node:undefined
console.log("varAge: ", globalThis.varAge);                             //Node:undefined Browser: Defined //Browser Output: 12
console.log("oopglobal: ", globalThis.oopsglobal);                      //Node: 10, Browser: 10 - //defined both
              

Sample execution command- NodeJS CommonJS javascript code

$> node script.cjs

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Sample Output - NodeJS CommonJS javascript code

Sample program output - part1

Sample program output - part2

Javascript in the browser

Sample Scenario

HTML reference, javascript code inclusion
- not to include the script with type = "module"
- Javascript as "module" has default use strict;
- This will make the program run with Strict Mode
- Variables without declaration are not allowed in the Strict Mode

Sample Code - In the Browser (Not a module) javascript code

script.js
index.html
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                  let letName = `John Doe`;
var varAge = 12;

function testoops() {
    // no declaration var means global var
    oopsglobal = 10;
}

function testlocal() {
    let local = oopsglobal;
    console.log("The Oopsglobal inside function: ", local);             // Result 10 - accessed within function undeclared
    return local;
}

testoops();
value = testlocal();

console.log("The Oops global using returned value is: ", value);        //Result 10 - return from local function
console.log("The Oops global using Global value is: ", oopsglobal);     //Result: 10 - Global usage oops global

console.log("This Value: ", this);                                      //Same as window, Output: {} for nodejs
//console.log("Global Value: ", global);                                //Browser JS : global not defined
console.log("GlobalThis Value: ", globalThis);                          //Same as window
console.log(window)                                                     //Same as globalThis, in browser (non NodeJS)

console.log("letName: ",globalThis.letName);                            //Node:undefined
console.log("varAge: ", globalThis.varAge);                             //Node:undefined Browser: Defined //Browser Output: 12
console.log("oopglobal: ", globalThis.oopsglobal);                      //Node: 10, Browser: 10 - //defined both
                  <!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>


  <script src="script.js"></script>
</body>

</html>
                

Sample Output - NodeJS CommonJS javascript code

Sample program output - Global This in the browser

Javasript - Debugger

Javasript - String