I know global variables are created when they are declared outside a function (says W3Schools).
If I create a global variable and edit it in a function, does it become local? Does the new value given by the function become the global value?
2
votes
4 Answers
8
votes
In general, no, editing a global does not make it local:
var myglob = 5;
function incGlob() {
myglob = myglob + 1;
}
incGlob();
console.log(myglob); // is 6 now
However, if you pass the global variable as an argument, the argument is a local copy:
var myglob = 5;
function incArg(myloc) {
myloc = myloc + 1;
}
incArg(myglob);
console.log(myglob); // is still 5
Note that objects are passed by reference, so editing the member variables of an argument variable changes the member variables of the original object passed in:
var myglob = { foo:5 };
function editLoc(myloc) {
myloc.foo = 6;
}
editLoc(myglob);
console.log(myglob.foo); // foo is 6 now
Finally, note that the local variable in editLoc, above, is just a reference. If we try to overwrite the entire object (instead of a member variable), the function simply loses the reference to the original object:
var myglob = { foo:5 };
function clobberLoc(myloc) {
myloc = { bar:7 };
}
clobberLoc(myglob);
console.log(myglob.foo); // myglob is unchanged...
// ...because clobberLoc didn't alter the object,
// it just overwrote its reference to the object stored in myglob
5
votes
No, editing the global variable will not change the variable's scope. The new value assigned becomes the global value.
myGlobal = 'foo'; // notice no 'var' keyword, implicitly global (DON'T DO THIS)
console.log(myGlobal); // logs 'foo'
var myFunc = function () {
myGlobal = 'bar';
};
myFunc();
console.log(myGlobal); // logs 'bar'
3
votes
