- A strict comparison (e.g. ===) is only true if the operands are of the same type.
- The more commonly used abstract comparison (e.g. ==) converts the operands to the same Type before making the comparison.
- For relational abstract comparisons (e.g., <=), the operands are first converted to primitives, then to the same type, before comparison.
- Strings are compared based on standard lexicographical ordering, using Unicode values.
Features of comparisons:
- Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
- Two numbers are strictly equal when they are numerically equal (have the same number value). NaN is not equal to anything, including NaN. Positive and negative zeros are equal to one another.
- Two Boolean operands are strictly equal if both are true or both are false.
- Two distinct objects are never equal for either strict or abstract comparisons.
- An expression comparing Objects is only true if the operands reference the same Object.
- Null and Undefined Types are strictly equal to themselves and abstractly equal to each other.
x == y
1 == 1 // true "1" == 1 // true 1 == '1' // true 0 == false // true 0 == null // false 0 == undefined // false null == undefined // true
x != y
1 != 2 // true 1 != "1" // false 1 != '1' // false 1 != true // false 0 != false // false
Identity / strict equality (===)
The identity operator returns true if the operands are strictly equal with no type conversion .
x === y
3 === 3 // true 3 === '3' // false
Non-identity / strict inequality (!==)
The non-identity operator returns true if the operands are not equal and/or not of the same type .
x !== y
3 !== '3' // true 4 !== 3 // true
Greater than operator (>)
The greater than operator returns true if the left operand is greater than the right operand.
x > y
4 > 3 // true
Greater than or equal operator (>=)
The greater than or equal operator returns true if the left operand is greater than or equal to the right operand.
x >= y
4 >= 3 // true 3 >= 3 // true
Less than operator (<)
The less than operator returns true if the left operand is less than the right operand.
x < y
3 < 4 // true
Less than or equal operator (<=)
The less than or equal operator returns true if the left operand is less than or equal to the right operand.
x <= y
3 <= 4 // true
You can find more information at MDN.
Comparing null and undefined
When we compare null and undefined we see different behaviour. Lets check different scenario through examples
Example - strict equality check (===)
console.log( null === undefined ); // O/P - false
Otuput is false and that’s correct because we know “null” and “undefined” are different types.
Example - non-strict equality check (==)
console.log( null == undefined ); // O/P: - true
How? This is because there is a special rule for “null” and “undefined”. Due to which they are equal in case of non-strict check (==) but are not equal to any other value.
If we use comparison operators like <, >, <=, >= etc., “null” and “undefined” are converted to number and in such cases “null” will become 0 and “undefined” will be NaN. Lets check some of those examples.
Example - compare null with 0 (zero)
console.log( null > 0 ); // O/P - false console.log( null >= 0 ); // O/P - true console.log( null == 0 ); // O/P - false
Strange? As per the first statement null is not greater than 0 and from the second statement null is either greater than or equal to 0. So, if we think mathematically and compare both statements, will come to the result that null is equal to 0. But, as per third statement it’s not true. Why?
The reason is “comparison” and “equality check” both works in different way. In comparison, “null/undefined” is first converted to number so, in first two cases “null” become 0 and hence case1) (null > 0) -> false and case2) (null >= 0) -> true. But, in equality check (==), “null/undefined” works without any conversion and as explained above (special rule), in equality check “null/undefined” are only equal to each other and are not equal to anything else. Hence (null == 0) -> false.
Example - compare undefined with 0 (zero)
console.log( undefined > 0 ); // O/P - false console.log( undefined >= 0 ); // O/P - false console.log( undefined == 0 ); // O/P - false
Here, we test the same cases as we did for null but again result is different. Why?
The reasons are as follow. In first two cases, we are comparing undefined with 0 and as mentioned above in comparison undefined gets converted to NaN. NaN is a special value which always return false when compared with any number and that’s why we got false as output in first two cases. For third statement, reason is same as mentioned for “null”. In equality check “null/undefined” are only equal to each other and not equal to anything else. Hence (undefined == 0) -> false.