How we can use the element or object or variables declared in one script in another script?

This is for using functions created in one script and using it in another.

From the functions also if we want to use the generated output in the calling do we need to return the variable and in the calling function assign it to another variable?

function a()

{

let value = 10 +3;

 return value;

}

function main()

{

var c = a();

}

Is there any way where we can use the generated output as it is in the calling function like global variable

Also want to use elements, objects and variables from one script in another script what are the options for it?