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Javascript Comparison Operators != vs !== [duplicate]

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javascript

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.

like image 700
manny Avatar asked Dec 23 '11 13:12

manny


People also ask

What is the difference between !== and != In JavaScript?

means that two variables are being checked for both their value and their value type ( 8!== 8 would return false while 8!== "8" returns true). != only checks the variable's value ( 8!=

Why do we prefer === and !== Over == and != In JavaScript?

The strict equality operator ( === ) behaves identically to the abstract equality operator ( == ) except no type conversion is done, and the types must be the same to be considered equal. The == operator will compare for equality after doing any necessary type conversions.

Should I use != Or !==?

Inequality Operators: !=!= : Converts values if variables are different types before checking for inequality. !== : Checks both type and value for the two variables being compared.


2 Answers

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:

  1. If Type(x) is different from Type(y), return false.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. 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.

  5. 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.

  6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  7. Return true if x and y refer to the same object. Otherwise, return false. NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

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:

  1. 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.

  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
  5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
  6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  8. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y) .
  9. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
  10. Return false
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Filip Roséen - refp Avatar answered Sep 19 '22 14:09

Filip Roséen - refp


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.

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Alnitak Avatar answered Sep 16 '22 14:09

Alnitak