Is a JavaScript variable automatically global if not declared with var?

Question

Is a JavaScript variable automatically global if not declared with var?

Here is the code snippet from one of my files:

function refreshGridSuccess(responseText, entity) {

        oTable = $('#dataTable').dataTable({
            "sScrollX": "100%",

In a different file, I have the following code:

$('#detailData')
            .on('click', '.sort-up', function (event) {
                event.preventDefault();
                var column = $(this).closest('th'),
                    columnIndex = column.parent().children().index(column.get(0));
                oTable.fnSort([[columnIndex, 'asc']]);
                return false;
            })

I have not explicitly defined the variable oTable anywhere else. The scripts are functioning correctly, so does this mean that oTable was automatically treated as a global variable?

Now, as I transition to using TypeScript, it prompts me to declare Otable before using it. Is there a method to declare oTable as an object, or do I need to specify its type based on what is returned by $('#dataTable').dataTable({}) ?

javascript jquery typescript

Answer 1

Answer №1

In the context of the global scope, there is no discernible difference between using "var" and not using it. However, within a function, declaring a variable with "var" will create a local variable while omitting it will search the scope chain for the variable or declare it in the global scope if it doesn't already exist.

Referenced from: Exploring the var keyword: its purpose and when to utilize it (or exclude it)

Answer 2

In the context of the global scope, there is no discernible difference between using "var" and not using it. However, within a function, declaring a variable with "var" will create a local variable while omitting it will search the scope chain for the variable or declare it in the global scope if it doesn't already exist.

Referenced from: Exploring the var keyword: its purpose and when to utilize it (or exclude it)

Answer 3

Answer №2

Indeed, a variable that is not declared with the var keyword will become global in scope. While it may work outside of a function without being explicitly declared, using globally defined variables created within functions can lead to unexpected and undefined behavior.

"It is important to note that failing to declare variables properly can result in unforeseen consequences. In ECMAScript 5 strict mode, attempting to assign a value to an undeclared variable inside a function will trigger an error." - https://example.com/learn-more-about-javascript

Answer 4

Indeed, a variable that is not declared with the var keyword will become global in scope. While it may work outside of a function without being explicitly declared, using globally defined variables created within functions can lead to unexpected and undefined behavior.

"It is important to note that failing to declare variables properly can result in unforeseen consequences. In ECMAScript 5 strict mode, attempting to assign a value to an undeclared variable inside a function will trigger an error." - https://example.com/learn-more-about-javascript

Answer 5

Answer №3

Indeed, you are accurate. In JavaScript, a variable without a var declaration is considered global and has the scope of window when in a browser environment.

It's important to understand that JavaScript variables are function-scoped. For example:

function test()
{
   for (var key in someObject)
   {
      var foo = 'bar';
   }
}

This code snippet can be rewritten as:

function test()
{
   var foo, key;
   for (key in someObject)
   {
      foo = 'bar';
   }
}

Regardless of where the var keyword appears within a block, the actual variable declaration takes place at the beginning of the function. This concept is known as variable hoisting, which you can research further online.

As for TypeScript, I haven't had the opportunity to experiment with it yet. However, I assume that you can still declare a variable like var oTable; just before using it, similar to JavaScript.

Answer 6

Indeed, you are accurate. In JavaScript, a variable without a var declaration is considered global and has the scope of window when in a browser environment.

It's important to understand that JavaScript variables are function-scoped. For example:

function test()
{
   for (var key in someObject)
   {
      var foo = 'bar';
   }
}

This code snippet can be rewritten as:

function test()
{
   var foo, key;
   for (key in someObject)
   {
      foo = 'bar';
   }
}

Regardless of where the var keyword appears within a block, the actual variable declaration takes place at the beginning of the function. This concept is known as variable hoisting, which you can research further online.

As for TypeScript, I haven't had the opportunity to experiment with it yet. However, I assume that you can still declare a variable like var oTable; just before using it, similar to JavaScript.

Is a JavaScript variable automatically global if not declared with var?

Answer №1

Answer №2

Answer №3

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