javascript 权威指南 学习札记2: Comparison Operators
日期：2014-05-16 浏览次数：20502 次
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Comparison Operators
The most commonly used types of relational operators are the comparison operators, which are used to determine the relative order of two values. The comparison operators are:
The < operator evaluates to true if its first operand is less than its second operand; otherwise it evaluates to false .
The > operator evaluates to true if its first operand is greater than its second operand; otherwise it evaluates to false .
The <= operator evaluates to true if its first operand is less than or equal to its second operand; otherwise it evaluates to false .
The >= operator evaluates to true if its first operand is greater than or equal to its second operand; otherwise it evaluates to false .
The operands of these comparison operators may be of any type. Comparison can be performed only on numbers and strings, however, so operands that are not numbers or strings are converted. Comparison and conversion occur as follows:
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If both operands are numbers, or if both convert to numbers, they are compared numerically.
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If both operands are strings or convert to strings, they are compared as strings.
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If one operand is or converts to a string and one is or converts to a number, the operator attempts to convert the string to a number and perform a numerical comparison. If the string does not represent a number, it converts to NaN , and the comparison is false . (In JavaScript 1.1, the string-to-number conversion causes an error instead of yielding NaN .)
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If an object can be converted to either a number or a string, JavaScript performs the numerical conversion. This means, for example, that Date objects are compared numerically, and it is meaningful to compare two dates to see whether one is earlier than the other.
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If the operands of the comparison operators cannot both be successfully converted to numbers or to strings, these operators always return false .
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If either operand is or converts to NaN , the comparison operator always yields false .
Keep in mind that string comparison is done on a strict character-by-character basis, using the numerical value of each character from the Unicode encoding. Although in some cases the Unicode standard allows equivalent strings to be encoded using different sequences of characters, the JavaScript comparison operators do not detect these encoding differences; they assume that all strings are expressed in normalized form. Note in particular that string comparison is case-sensitive, and in the Unicode encoding (at least for the ASCII subset), all capital letters are "less than" all low
