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I am using lint and for the following:

if (json.RowKey != json.NewRowKey)

It gives me a message:

Expected '!==' and instead saw '!='.

Can someone explain what this means?

  • 2
    "Crockford said that using double-equals (==) to compare objects is always a bad idea because it does implicit type conversion." [0](http://osric.com/chris/accidental-developer/2012/01/douglas-crockford-programming-style-and-your-brain/) [1](http://shop.oreilly.com/product/9780596517748.do) – pdoherty926 Sep 25 '12 at 05:29

4 Answers4

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== will attempt to convert the types of the two operands to be the same before comparing them. Thus "2" == 2 is true.

=== will not attempt to convert the types of the two operands so if they are not the same type, they will never be ===. Thus "2" === 2 is false.

It is better to use === and !== as your default choice and only use == or != when you explicitly want to allow type conversion. If you are not expecting the two operands to be different types, then === and !== is more likely to do what you are expecting.

This avoids coding mistakes when things are not really equal and your code doesn't intend for them to be equal.

Some odd things that happen with == (and do not happen with ===) that you may not expect and can lead to trouble. All of these evaluate to true:

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

There's more detail written here: Which equals operator (== vs ===) should be used in JavaScript comparisons?

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jfriend00
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!== is not equal (neither value or type) Eg: var x = 5; x !== "5" returns true var x = 5; x !== 5 returns false

thomasbabuj
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It's a warning that tells you that you should consider using the !== operator instead of !=.

On the surface they both do the same thing - compare whether the two items are not equal. However, !== makes sure that both items are of the same type first (that is, it won't try to convert, say, a string to a number if one item being compared is a string and the other is a number), so it's generally safer to use and helps you avoid comparison errors.

For more details, see here: Which equals operator (== vs ===) should be used in JavaScript comparisons?

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Adam Lear
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Yes,

if("1" == 1) //Returns true because it will convert the second 1 into a string to match

but 

"1" !== 1 //Returns false because the first 1 is a string and the second 1 is an integer.

We are talking about variable types; with two equal signs, javascript (and php) will typecast the variable for you (convert the integer 1 into a string 1 to make the match and be true), but with the second statement, it will not.

1 === 1 //True
1 === "1" //False
1 == "1" //True
1 == 1 //True

Hope my answer makes sense to you.

Layton Everson
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