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Chaining assignments in var declarations.

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Chaining assignments as part of a var declaration will create global variables unintentionally.

For example: 

(function foo() {    
    var a = b = 0;
})()
console.log('a: ' + a);
console.log('b: ' + b);

Will result in: 

Uncaught ReferenceError: a is not defined
'b: 0'

In the above example, a is local but b becomes global. This is because of the right to left evaluation of the = operator. So the above code actually evaluated as 

var a = (b = 0);

The correct way to chain var assignments is: 

var a, b;
a = b = 0;

Or: 

var a = 0, b = a;

This will make sure that both a and b will be local variables.

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* Chaining assignments in var declarations.
Table Of Contents
0 Getting started
1 Comments
2 Basic data types
3 Built-in constants
4 Variables
5 Declarations and assignments
6 Reserved keywords
7 Data types in JavaScript
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48 Timestamps
49 Destructuring assignment
50 Web Worker
51 Debugging
52 Notifications API
53 WebSockets
54 Web Cryptography API
55 Async functions, async / await
56 Constructor functions
57 execCommand and contenteditable
58 Performance Tips
59 Design patterns
60 requestAnimationFrame
61 Method chaining
62 Global error handling in browsers
63 File API, blobs and FileReaders
64 Console
65 Tail call optimization
66 Detecting browser
67 Enumerations
68 Symbols
69 Localization
70 Selection API
71 Callbacks
72 Functional JavaScript
73 Modals
74 Data attributes
75 Event Loop
76 Events
77 Data manipulation
78 BOM Browser Object Model
79 Unit Testing
80 Linters for ensuring code quality
81 Automatic Semicolon Insertion
82 IndexedDB
83 Anti-patterns
84 Navigator Object
85 Modularization techniques
86 Proxy
87 Same origin policy, cross-origin communication
88 postMessage and MessageEvent
89 WeakMap
90 WeakSet
91 Behaviroal Design Patterns
92 Server-sent events
93 Async iterators
94 Namespacing
95 Evaluating JavaScript
96 Memory efficiency
97 How to make iterator usable inside async callback function
98 Context this
99 Setters and getters
100 Vibration API
101 Fluent API
102 Tilde
103 Security issues
104 Get and set CSS custom variables
105 Contributors