JSLint Expected '===' and instead saw '=='

Recently I was running some of my code through JSLint when I came up with this error. The thing I think is funny about this error though is that it automatically assumes that all == should be ===.

Does that really make any sense? I could see a lot of instances that you would not want to compare type, and I am worried that this could actually cause problems.

The word "Expected" would imply that this should be done EVERY time.....That is what does not make sense to me.

IMO, blindly using ===, without trying to understand how type conversion works doesn't make much sense.

The primary fear about the Equals operator == is that the comparison rules depending on the types compared can make the operator non-transitive, for example, if:

A == B AND
B == C

Doesn't really guarantees that:

A == C

For example:

'0' == 0;   // true
 0  == '';  // true
'0' == '';  // false

The Strict Equals operator === is not really necessary when you compare values of the same type, the most common example:

if (typeof foo == "function") {
  //..
}

We compare the result of the typeof operator, which is always a string, with a string literal...

Or when you know the type coercion rules, for example, check if something is null or undefinedsomething:

if (foo == null) {
  // foo is null or undefined
}

// Vs. the following non-sense version:

if (foo === null || typeof foo === "undefined") {
  // foo is null or undefined
}

JSLint is inherently more defensive than the Javascript syntax allows for.

From the JSLint documentation:

The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors.

When comparing to any of the following values, use the === or !== operators (which do not do type coercion): 0 '' undefined null false true

If you only care that a value is truthy or falsy, then use the short form. Instead of

(foo != 0)

just say

(foo)

and instead of

(foo == 0)

say

(!foo)

The === and !== operators are preferred.

Keep in mind that JSLint enforces one persons idea of what good JavaScript should be. You still have to use common sense when implementing the changes it suggests.

In general, comparing type and value will make your code safer (you will not run into the unexpected behavior when type conversion doesn't do what you think it should).