64 lines
2.5 KiB
Markdown
64 lines
2.5 KiB
Markdown
---
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title: Global Variables
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---
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Global variables are declared outside of a function for accessibility throughout the program, while local variables are stored within a function using `var` for use only within that function's [scope](https://developer.mozilla.org/en-US/docs/Glossary/Scope). If you declare a variable without using `var`, even if it's inside a function, it will still be seen as global:
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```javascript
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var x = 5; //global variable
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function someThing(y) {
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var z = x + y;
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console.log(z);
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}
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function someThing(y) {
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x = 5; //still a global variable!
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var z = x + y;
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console.log(z);
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}
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function someThing(y) {
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var x = 5; //local variable
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var z = x + y;
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console.log(z);
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}
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```
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A global variable is also an object of the current scope, such as the browser window:
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```javascript
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var dog = “Fluffy”;
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console.log(dog); //Fluffy;
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var dog = “Fluffy”;
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console.log(window.dog); //Fluffy
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```
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It’s a best practice to minimize global variables. Since the variable can be accessed anywhere in the program, they can cause strange behavior.
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References:
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* [var -Javascript|MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
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* [You Don't Know JavaScript: Scopes & Closures](https://github.com/getify/You-Dont-Know-JS/tree/master/scope%20%26%20closures)
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Additional Info:
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* <a href='http://www.w3.org/wiki/JavaScript_best_practices#Avoid_globals' target='_blank' rel='nofollow'>JavaScript Best Practices: Avoid Globals</a>
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* [What's the difference between a global var and a window.variable in javascript?](https://stackoverflow.com/questions/6349232/whats-the-difference-between-a-global-var-and-a-window-variable-in-javascript)
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---
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The scope of JavaScript variables are either global or local.
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Global variables are declared OUTSIDE the function and its value is accessible/changeable throughout the program.
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You should ALWAYS use **var** to declare your variables (to make locally) else it will install GLOBALLY
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Take care with the global variables because they are risky. Most of the time you should use closures to declare your variables.
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Example:
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```javascript
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(function(){
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var myVar = true;
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})();
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```
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#### More Information:
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- [JavaScript Best Practices: Avoid Globals](http://www.w3.org/wiki/JavaScript_best_practices#Avoid_globals)
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- [Global Variables are bad](http://c2.com/cgi/wiki?GlobalVariablesAreBad)
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- [MDN - Global Variables](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
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