In Javascript everyone recommends to use strict equality instead of lenient. In this article we will see the difference between strict and lenient equality and discover why it should not be used. So lets begin.
2 == true // 2 === 1 false 2 == false // 2 === 0 false 1 == true // 1 === 1 true 0 == false // 0 === 0 true
2 is not equal to true
because lenient equality tries to convert true to number which is 1 and then does strict equality therefore 2 is not equal to 1.
2 is not equal to false also
, why , because false is converted to 0 then 2 cannot be equal to 0.
1 is equal to true
because true is converted to 1 thus it is true. Similarly false is equal to 0.
'' == false // 0 === 0 true 'abc' == true // NaN === 1 false
Empty string is false because conversion of empty string to number is 0
Number('') // 0
0 is equal to 0 because false is also converted to 0.
'abc'
is converted to NaN by Number thus Nan is not equal to 1
.
'\n\t123\r ' == 123 true '' == 0 // 0 === 0 true
In first line comes the confusion \n\t123\r
is also converted to 123
. Thus it becomes true.
In second line its understandable with previous examples.
{} == '[object Object]' true ['123'] == 123 true [] == 0 true
In first case lenient equality tries to convert it to string i.e. String({})
which gives '[object Object]'
. Thus it is true.
In Second case, again it convert to string String(['123']) = 123
. Thus it also becomes equal.
In third case, agains come the problem this time it uses Number conversion Number([]) = 0
thus it also satisfies.
Confusing? Lets see how these works.
The algorithm for comparing via normal equality works as follows. If both operands have the same type (one of the six specification types—Undefined, Null, Boolean, Number, String, and Object)
, then compare them via strict equality.
Otherwise, if the operands are:

undefined
andnull
, then they are considered leniently equal:undefined == null true
 A string and a number, then convert the string to a number and compare both operands via strict equality.
 A boolean and a nonboolean, then convert the boolean to a number and compare leniently (again).
 An object and a number or a string, then try to convert the object to a primitive (via the algorithm described in Algorithm: ToPrimitive()—Converting a Value to a Primitive) and compare leniently (again).
Otherwise—if none of the aforementioned cases apply—the result of the lenient comparison is false
.
This is taken from http://speakingjs.com/es5/ch09.html.
Now try finding the reasons for below and comment for others.
undefined==null true undefined===null false
Try everything in console and explore.
Also read about Javascript’ Unknown fact and magic here
https://www.learnsteps.com/javascriptincreasingperformancebyhandlingscopessmartly/
https://www.learnsteps.com/javascriptincreasingperformanceusingdynamicloading/
https://www.learnsteps.com/javascriptconfusingconceptsanduncertainities/
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1 COMMENT
awesome work sir . keep sharing your experience