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Tom.Reding (talk | contribs) m Ref cleanup; WP:GenFixes on, rep 2 space(s)+htab+space(s) with 1 space, rem 8 htabs-with-an-adjacent-space, rep 6 text-surrounded-htabs with spaces, spaces (1) |
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[[JavaScript]] provides a [[Boolean data type]] with {{mono|true}} and {{mono|false}} literals. The {{mono|[[typeof]]}} operator returns the string {{mono|"boolean"}} for these [[primitive types]]. When used in a logical context, {{mono|0}}, {{mono|-0}}, {{mono|null}}, {{mono|NaN}}, {{mono|undefined}}, and the empty string ({{mono|""}}) evaluate as {{mono|false}} due to automatic [[type coercion]]. All other values (the [[complement (set theory)|complement]] of the previous list) evaluate as {{mono|true}}, including the strings {{mono|"0"}}, {{mono|"false"}} and any object. Automatic type coercion by the equality comparison operators (<code>==</code> and <code>!=</code>) can be avoided by using the type checked comparison operators (<code>===</code> and <code>!==</code>).
When type conversion is required, JavaScript converts {{mono|Boolean}}, {{mono|Number}}, {{mono|String}}, or {{mono|Object}} operands as follows:<ref>{{cite web | url=https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators | title=Comparison Operators - MDC Doc Center | publisher=Mozilla | date=5 August 2010 |
;{{small|Number and String}}: The string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.
;{{small|Boolean}}: If one of the operands is a Boolean, the Boolean operand is converted to 1 if it is {{mono|true}}, or to 0 if it is {{mono|false}}.
;{{small|Object}}: If an object is compared with a number or string, JavaScript attempts to return the default value for the object. An object is converted to a primitive String or Number value, using the {{mono|.valueOf()}} or {{mono|.toString()}} methods of the object. If this fails, a runtime error is generated.
[[Douglas Crockford]] advocates the terms "truthy" and "falsy" to describe how values of various types behave when evaluated in a logical context, especially in regard to edge cases.<ref>{{cite web | url=http://javascript.crockford.com/style2.html | title=The Elements of JavaScript Style | publisher=Douglas Crockford |
The binary logical operators returned a Boolean value in early versions of JavaScript, but now they return one of the operands instead. The left–operand is returned, if it can be evaluated as : {{mono|false}}, in the case of [[logical conjunction|conjunction]]: (<code>a && b</code>), or {{mono|true}}, in the case of [[logical disjunction|disjunction]]: (<code>a || b</code>); otherwise the right–operand is returned. Automatic type coercion by the comparison operators may differ for cases of mixed Boolean and number-compatible operands (including strings that can be evaluated as a number, or objects that can be evaluated as such a string), because the Boolean operand will be compared as a numeric value. This may be unexpected. An expression can be explicitly cast to a Boolean primitive by doubling the logical [[negation|negation operator]]: ({{mono|!!}}), using the {{mono|Boolean()}} function, or using the [[Conditional (programming)|conditional operator]]: (<code>c ? t : f</code>).
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