Understanding double and triple equals in Javascript

== (double equal) is also referred as loose equality. It allows coercion when comparing two values for equality

14 == '14'  //true

=== (triple equal) is also referred as strict equality.It does not allow coercion.

14 === '14'  //false

== VS ===

It is a misconception that double equal operator checks for value only and triple equal operator checks for value and type. They both checks for type. (Refer to spec 7.2.14Abstract Equality Comparison)

"donut" == "donut" //true

"donut" === "donut"  //true

double and triple equal operator behaves exactly same when when types match because spec says when types are same double equal returns the result of triple equal comparison (strict equality comparison)

There are 3 types of coercion: Number, String and Boolean.

Number Conversion

Number(undefined)    //NaN

Number(null)              // 0

Number(true)            //  1

Number(false)            // 0

Number(NaN)           // NaN

Number('44')             // 44

Number('apple')        // NaN

Number("")              // 0

Number([])                 // 0

Number([44])             // 44

Number(["44"])          // 44

Number(["1","2"])      // NaN

Number(["apple"])     // NaN

Number({})               // NaN

Number(Symbol())    // throws TypeError

String Conversion

String(undefined) // "undefined"

String(null) // "null"

String(true) // "true"

String(false) // "false"

String(NaN) // "NaN"

String(44) // "44"

String([]) //""

String(["44"]) // "44"

String(["44","apple"]) //"44, apple"

String({})  // "[object Object]"

Boolean Conversion

Boolean(undefined) // false

Boolean(null) // false

Boolean(false) // false

Boolean(true) // true

Boolean(“44”) // true

Boolean(“”) // false

Boolean(44) // true

Boolean(0) // false

Boolean(-0) // false

Boolean(NaN) // false

Boolean([]) // true

Boolean(["44"]) // true

Boolean(["44","apple"]) // true

Boolean({}) // true

Boolean(Symbol()) // true

There are six falsy values: undefined, null, false, “”(empty string), 0(number zero) and NaN.

Double equal comparison (Abstract Equality Comparison)

It prefers to do numeric comparison. If one variable is number and other variable is string or boolean, it coerces string and boolean to number.

44 == "44"  // true

 // "44" is coerce to number and compared

true == 1    // true

false == 1   // false

Here boolean value is coerced to number and compared.

If one variable is either number, string, or symbol and other is object, it turns object(non primitive value) to primitive by invoking toPrimitive abstract operation.

"apple" == {a: "apple"}    //false

"apple" == String({a: "apple"})  // [object Object]

"apple" == [object Object] // false

Here object is coerced to string and compared.

44 == [44]    //true

44 == Number[44]   //44

44 == 44  //true

Here array is coerced to number and compared.

"44" == true   //false

Number(44) == Number(true)

44 == 1    //false

Here string and boolean are coerced to number and compared.

null and undefined

undefined == undefined    //true

undefined == null    //true

null == null    //true

null == undefined   // true

Null and undefined are only true to themselves and each other. They will return false if compared to other value.

NaN

NaN == NaN   //false

Any value is compared to NaN including NaN itself returns false.

Triple Equal Operator(Strict Equality Comparison)

If you compare value of different type, it will return false.

If type is same but value is different, it will return false.

"44" === 44    //false

(different type)

"batman" === "superman"   //false

(same type but different value)

44 === 44    //true

(same type and value)

null === undefined     //false

(different type but returns true in double equal comparison)

NaN === NaN     //false

(Any value compared to NaN always return false including itself)