CODEWITHSUNDEEP
javascriptstrict
user4385532
user4385532

Reputation:

What does JavaScript's strict mode exactly do regarding implicit global declarations?

From MDN article about strict mode:

First, strict mode makes it impossible to accidentally create global variables. In normal JavaScript mistyping a variable in an assignment creates a new property on the global object and continues to "work" (although future failure is possible: likely, in modern JavaScript). Assignments, which would accidentally create global variables, instead throw an error in strict mode:

'use strict';
                       // Assuming a global variable mistypedVariable exists
mistypeVariable = 17;  // this line throws a ReferenceError due to the 
                       // misspelling of variable

What does this mean? Does the engine detect if a variable with a similar name already exists(?!) or does strict mode merely prohibit declaring global variables outside of global scope?

The above quotation seems to hint the first possibility but this seems... weird?

Upvotes: 2

Views: 1327

Answers (3)

Robin Zigmond
Robin Zigmond

Reputation: 18249

As requested, I'll make this into an answer :)

I think you understand what happens here already, and are just getting confused through reading that quote about "assuming a global variable..." too literally. (I will admit it is phrased in such a way as to possibly lead to this confusion though.) The real situation is very simple, and has nothing to do with variables having "similar names" (which JS has no concept of):

What is being talked about is what happens if you assign a value to a variable which has not been formally declared (variables are declared with one of the keywords var, let or const). Not declaring your variables is bad practice, and in strict mode will throw an error - this is a good thing and warns you of your mistake. But in non-strict mode, JS will happily accept this and think you wanted to declare a global variable of that name. This is almost never what you actually want, as it pollutes the global namespace, doesn't inform you of your error, and may lead to all sorts of tricky bugs later.

Upvotes: 2

mrReiha
mrReiha

Reputation: 955

does strict mode merely prohibit declaring global variables outside of global scope?

strict mode definitely doesn't prohibit declaring global variables from anywhere. In non-strict mode, if you write:

someVar = 't';

it will evaluate to:

window.someVar = 't';

( why is this happening? ) despite of writing inside or outside of a function scope. actually, the line was both declaring and evaluation of variable ( look again, it doesn't have var so it shouldn't declare anything! ).

But it would cause such a side-effect that wasn't totally fine and they've introduced strict-mode which when is active, our first line will throw an error. because it's just evaluation without declaring it first.

now if you need to manipulate global scope inside of a function scope, you only should global object as reference:

var someGlobalVar;
var someOtherGlobalVar;

function hello() {

    // this will make *someGlobalVar* to be redefined in the inner scope
    // someGlobalVar != window.someGlobalVar
    var someGlobalVar;

    // manipulating inner variable, not global one
    someGlobalVar = 's';


    // it's fine, you've accessed global object correctly
    window.someGlobalVar = 's';


    // you're still editing global object, because
    // there's no other variable inside of this scope having the same name
    someOtherGlobalVar = 's';

}

Upvotes: 0

trincot
trincot

Reputation: 350310

The comment in the quoted code is misleading: The "Assuming" part is irrelevant. It should really read: "Assuming that no global variable with exactly this name was defined with let or var,..."

But do note what the purpose was of the code snippet and the comment: to show that in strict mode it is much easier to spot spelling mistakes. In sloppy mode the spelling mistake would go unnoticed (no errors) and you would be working with two variables without knowing it (at least for a while)

Upvotes: 2

Related Questions