javascript - Type casting when == used instead of === -

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this question has answer here:

i don't prefer using == today experimenting following code including == , results bit confusing me. can please explain happening?

all these falsy values: 

'', 0, false, undefined, null

suppose did:

if (undefined == null) {     alert('a'); } else {     alert('b'); }

the statements below give true:

null == undefined 0 == '' false == '' 0 == false

but why code below return false?

undefined == 0 undefined == '' null == 0

because 0, false , '' "values" while null "absence of value" , undefined "absence of definition".

thus, can compare "values" each other, same "non-values". can't compare "values" "non-values".


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