Javascript Comparison Operators != vs !==

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

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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, thena. 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.

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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), thena. If Type(x) is Undefined, return t rue.b. If Type(x) is Null, return true.c. If Type(x) is Number, then1. 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