JavaScript – Does Using == Ever Make Sense?

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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 === produces true and !== produces false. 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     // true

The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.

Given this unequivocal observation, is there ever a time when using == might actually be appropriate?

Best Answer

I'm going to make an argument for ==

Douglas Crockford which you cited is known for his many and often very useful opinions. While I'm with Crockford in this particular case it's worth mentioning it is not the only opinion. There are others like language creator Brendan Eich who don't see the big problem with ==. The argument goes a little like the following:

JavaScript is a behaviorally* typed language. Things are treated based on what they can do and not their actual type. This is why you can call an array's .map method on a NodeList or on a jQuery selection set. It's also why you can do 3 - "5" and get something meaningful back - because "5" can act like a number.

When you perform a == equality you are comparing the contents of a variable rather than its type. Here are some cases where this is useful:

  • Reading a number from the user - read the .value of an input element in the DOM? No problem! You don't have to start casting it or worrying about its type - you can == it right away to numbers and get something meaningful back.
  • Need to check for the "existence" of a declared variable? - you can == null it since behaviorally null represents there is nothing there and undefined doesn't have anything there either.
  • Need to check if you got meaningful input from a user? - check if the input is false with the == argument, it will treat cases the user has entered nothing or just white-space for you which is probably what you need.

Let's look at Crockford's examples and explain them behaviorally:

'' == '0'           // got input from user vs. didn't get input - so false
0 == ''             // number representing empty and string representing empty - so true
0 == '0'            // these both behave as the number 0 when added to numbers - so true    
false == 'false'    // false vs got input from user which is truthy - so false
false == '0'        // both can substitute for 0 as numbers - so again true

false == undefined  // having nothing is not the same as having a false value - so false
false == null       // having empty is not the same as having a false value - so false
null == undefined   // both don't represent a value - so true

' \t\r\n ' == 0     // didn't get meaningful input from user vs falsey number - true

Basically, == is designed to work based on how primitives behave in JavaScript, not based on what they are. While I don't personally agree with this point of view there is definitely merit in doing it - especially if you take this paradigm of treating types based on behavior language-wide.

* some might prefer the name structural typing which is more common but there is a difference - not really interested in discussing the difference here.

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