Abstract Euality Comparison (==) vs Strict Equality Comparison (===)

== and === both checks for type if the value's type matches both comparison is same, but if types does not matches == allows coercion while === does not.

Is coercion helpful in an equality comparison or not?

The ESLint rule eqeqeq prevents the use of == (Abstract Equality Comparison). Whether this rule is helpful its a decision of your team. However, it’s important to understand when and how to use == effectively, as it can help you avoid unnecessary bugs caused by unexpected type coercion.

Abstract Equality Comparsion

Abstract Equality Comparsion is same when the values type matches. If Type(x) is same as Type(y) then it return the result of performing Strict Equality Comparison x===y.

• If x is null and y is undefined, return true.

• If x is undefined and y is null, return true.

If only if Type(x) and Type(y) does not matches, Abstract Equality Comparison allow coercion.

• If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).

• If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.

• If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

• If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

• If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).

• If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.

Corner cases

42 == [42]                  // true

42 == "42"
42 == 42 

[] == ![]                   // true

// [] is truthy value so it evaluates to true and !true is false
[] == false
// because one value is non-ptimitive ToPrimitive will be applied
"" == false
// Abstract equality conversion prefer ToNumber
0 == 0

var students = []           // true

// This is correct way of checking..
if(students) {
    // Yep
}

// [] == true
// "" == true
// 0 == 1
if(students == true) {
    // Nope
}

// [] == false
// "" == false
// 0 == 0
if(students == false) {
    // Yep
}

Strict Equality Comparison

Strict Equality Comparison return false if the types different for x and y.

• undefined === undefined return true
• null === null return true
• NaN === NaN return false
• +0 === -0 return true
• -0 === 0 return true