Why does JavaScript allow you to use variables without declaring?

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Why does JavaScript allow you to use variables without declaring?

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#1 by (6 votes)
#2 by (5 votes)
#3 by (2 votes)

6

I was testing the JSFiddle with the code below. In it I did not declare the variable i , but even so, I can use it normally.

Is this intentional or is it a failure? So, can I just quit using non-declared, Python-style variables?

I also realized that I can use the variable anywhere in the code, obviously this makes sense because as it is not declared, it has no scope. But is this "possibility" just to be able to create "global scope" variables?

for(i = 0; i < 10; i++){
    document.write(i + '<br>');
}

i = 'asdasd';

document.write(i);

javascript

asked by anonymous 13.12.2016 / 16:36

3 answers

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score 6 · Accepted Answer

6

If it is intentional or it is flawed there are controversies :) Well, it can be said that it is intentional. Like all "good" dynamic language , in every way, like a

13.12.2016 / 16:47

5

When you stop using var in a declaration of a variable, you are declaring it in the global context, independent of the scope in which you declared it.

See:

function call_me_baby() {
    a = 1;
    var b = 2;
}


call_me_baby();

console.log(typeof(a), typeof(b));

Note that the a variable has been "sent" to the global scope. The variable b was limited to the scope of call_me_baby .

When you make a declaration of a variable without using var , we could say that it is the equivalent of doing the assignment directly on the object window .

See:

call_me_baby() {
    window.a = 1;
}

In the specific case of for , the same thing would happen if you did not use var to declare the variable. The variable i would be implicitly defined in the global scope, regardless of the scope in which for is invoked.

You can even use var in for in two ways:

 var i;

 for (i = 0; i < 10; i++) {}

Or

 for  (var i = 0; i < 10; i++) {}

It is important to note that failure to declare i could cause problems with collision of names and undue declarations.

Another important note is that with the use of "use strict" , the lack of var in the i statement could generate an error.

(function () {
  "use strict";
  a = 1;
 })();

See more at:

When to use var in javascript?

13.12.2016 / 16:46

2

The language is not perfect :) Being able to make value assignments to undeclared variables is a language flaw that has poor typing. I say failure because it generates variable overrides without giving account and it generates hard-to-find errors.

Example:

(function(c) {
    console.log(a); // undefined
    try {
        console.log(b); // vai dar erro
    } catch (e) {
        console.log('Erro!'); // erro: ReferenceError: b is not defined
    }
    var a = 10;
    console.log(a); // 10
    b = 10;
    console.log(b); // 10
    console.log(c); // undefined
})();
console.log('No escopo global:')
console.log(typeof a, typeof b, typeof c); // b dá number!, as outras estão undefined

In this example above the variable b and c are not declared explicitly. b is exported to the global scope, which is dangerous and can generate ugly bugs. c is generated by the function, as a parameter. In a way it is stated, but not explicitly. This generates code interpretation errors and should be avoided as we may think that calling the function without passing a value as an argument is a bug.

Since there is a lot of code that trusts this (wrong) behavior, it is not unconventional to change this behavior.

How to avoid this?

The best way to avoid this kind of problem is to use strict mode . . strict mode is a command that causes a code to run in a more controlled mode. It requires that some important rules be fulfilled, one of which is the same that you speak in the question. That is: strict mode give values to undeclared variables gives error!

function normal() {
    a = 10;
}
normal();
console.log(typeof a); // number


function strict() {
    "use strict";
    b = 10; // o código pára aqui pois isto gera um erro
}
strict();
console.log(typeof b);

16.12.2016 / 09:11