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Global Object refers to the top object, which is window in browsers, while global in Node.js.

In ES5, the property of a Global Object is same as a Global Variable.

window.a = 1;
console.log(a);             /** => 1            */

a = 2;
console.log(window.a);      /** => 2            */

In ES6, variables defined by var and function are still same as Global Variables. However, variables defined by let, const or class do not belong to properties of the Global Object.

var a = 1;

/** in Node.js, it should be `global.a = 1;` or `this.a = 1;`  */
console.log(window.a);      /** => 1            */

let b = 1;
console.log(window.b);      /** => undefined    */