Avoid use of `==` and `!=`JS-0050

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The strict equality operators (=== and !==) use the strict equality comparison algorithm to compare two operands.

• If the operands are of different types, return false.
• If both operands are objects, return true only if they refer to the same object.
• If both operands are null or both operands are undefined, return true.
• If either operand is NaN, return false.
• Otherwise, compare the two operand's values:
  • Numbers must have the same numeric values. +0 and -0 are considered to be the same value.
  • Strings must have the same characters in the same order.
  • Booleans must be both true or both false.

The most notable difference between this operator and the equality (==) operator is that if the operands are of different types, the == operator attempts to convert them to the same type before comparing.

Bad Practice

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Recommended

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null