How to access same variable name inside function in javascript

Try :

var xyz = "Hello";
console.log("outside function: " + xyz)
function abc() {
    var xyz = (window.xyz || "") +" " + "World!";
    console.log("inside function: " + xyz) // output should be "Hello world!"
} abc();

if you do not want to edit the value of xyz:

var xyz = "Hello";
console.log("outside function: " + xyz)
function abc() {
    var abc = " World!";
    console.log("inside function: " + xyz + abc) // output should be "Hello world!"
} abc();

if you want to edit the value of xyz:

var xyz = "Hello";
console.log("outside function: " + xyz)
function abc() {
    xyz += " World!";
    console.log("inside function: " + xyz) // output should be "Hello world!"
} abc();

Try with window. This will allow you to access globally declared variable.

var xyz = window.xyz ..

Why not just pass it as a parameter? There is absolutely no need to use a global variable.

var xyz = "Hello";
console.log("outside function: " + xyz)
function abc(xyz) {
    var xyzUpdated = xyz + "World!";
    console.log("inside function: " + xyzUpdated) // output should be "Hello world!"
} abc(xyz);