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Logic Operators with Non-boolean values boolean coercion

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Logical OR (||), reading left to right, will evaluate to the first truthy value. If no truthy value is found, the last value is returned. 

var a = 'hello' || '';             // a = 'hello'
var b = '' || [];                  // b = []
var c = '' || undefined;           // c = undefined
var d = 1 || 5;                    // d = 1
var e = 0 || {};                   // e = {}
var f = 0 || '' || 5;              // f = 5
var g = '' || 'yay' || 'boo';      // g = 'yay'

Logical AND (&&), reading left to right, will evaluate to the first falsy value. If no falsey value is found, the last value is returned. 

var a = 'hello' && '';                  // a = ''
var b = '' && [];                       // b = ''
var c = undefined && 0;                 // c = undefined
var d = 1 && 5;                         // d = 5
var e = 0 && {};                        // e = 0
var f = 'hi' && [] && 'done';           // f = 'done'
var g = 'bye' && undefined && 'adios';  // g = undefined

This trick can be used, for example, to set a default value to a function argument (prior to ES6). 

var foo = function(val) {
    // if val evaluates to falsey, 'default' will be returned instead.
    return val || 'default';
}

console.log( foo('burger') );  // burger
console.log( foo(100) );       // 100
console.log( foo([]) );        // []
console.log( foo(0) );         // default
console.log( foo(undefined) ); // default

Just keep in mind that for arguments, 0 and (to a lesser extent) the empty string are also often valid values that should be able to be explicitly passed and override a default, which, with this pattern, they won’t (because they are falsy).

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Comparison operations:
* Abstract equality inequality and type conversion
* NaN Property of the Global Object
* Short-circuiting in boolean operators
* Double Negation (!!)
* Abstract equality
* Bitwise operators
* Null and undefined
* Automatic Type Conversions
* Logic Operators with Non-boolean values boolean coercion
* Empty Array
* Logic Operators with Booleans
* Equality comparison operations
* Inequality
* Relational operators
* Grouping multiple logic statements
* List of Comparison Operators
* Bit fields to optimize comparison of multi state data
Table Of Contents
0 Getting started
1 Comments
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3 Built-in constants
4 Variables
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15 Variable coercion / conversion
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88 postMessage and MessageEvent
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91 Behaviroal Design Patterns
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97 How to make iterator usable inside async callback function
98 Context this
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100 Vibration API
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102 Tilde
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104 Get and set CSS custom variables
105 Contributors