Javascript Comparison Operators != vs !==
Possible Duplicate:
Javascript === vs == : Does it matter which “equal” operator I use?
Difference between == and === in JavaScript
I have two variables to compare.
Result should not be equal, in which condition i need to use != and !== ?
because when i use both operator it is working properly, but i need to know exactly what is the difference.
Human readable text about their differences
Using !==
and ===
will do a more strict compare than ==
/ !=
. The former will check if the objects being compared are of the same type, as well as if the values matches.
Using ==
will make it possible for an implicit cast to be made, see the below examples.
(0 == '0') // true
(0 === '0') // false
('' == 0 ) // true, the string will implicitly be converted to an integer
('' === 0 ) // false, no implicit cast is being made
What does the standard say?
11.9.6 The Strict Equality Comparison
Algorithm The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
If Type(x) is Number, then
a. If x is NaN, return false.
b.If y is NaN, return false.
c. If x is the same Number value as y, return true.
d. If x is +0 and y is 0, return true.
e. If x is 0 and y is +0, return true.
f. Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
11.9.3 The Abstract Equality Comparison Algorithm
The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:
If Type(x) is the same as Type(y), then
a. If Type(x) is Undefined, return t rue.
b. If Type(x) is Null, return true.
c. If Type(x) is Number, then
1. If x is NaN, return false.
2. If y is NaN, return false.
3. If x is the same Number value as y, return true.
4. If x is +0 and y is 0, return true.
5. If x is 0 and y is +0, return true.
6. Return false.
d. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
e. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false. f. Return true if x and y refer to the same object. Otherwise, return false.
The difference is that the former ( !=
) version will coerce the two variables to be type compatible before the comparison. Hence:
"" == 0 -> true
"" === 0 -> false
The other version requires strict equality - the two values must both be of the same type and have the same value. Most of the time this is the one you should actually use.
In the case of objects strict equality means that they are actually the same object. A comparison between objects does not perform a field-by-field comparison of the contents of the object.
See https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators for more.
区别很简单:!==仅当变量具有相同类型且相等时才返回。
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