In Javascript, the Good Parts, Douglas Crockford wrote:
JavaScript has two sets of equality operators:
===and!==, and their evil twins==and!=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then===producestrueand!==producesfalse. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. The rules by which they do that are complicated and unmemorable. These are some of the interesting cases:'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // trueThe lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use
===and!==. All of the comparisons just shown producefalsewith the===operator.
Given this unequivocal observation, is there ever a time when using == might actually be appropriate?
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