为什么“undefined equals false”返回 false？

Question

When I compare undefined and null against Boolean false, the statement returns false:

undefined == false;
null == false;

It return false. Why?

Answer 1

With the original answer pointing to the spec being deleted, I'd like to provide a link and short excerpt from the spec here.

http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

The ECMA spec doc lists the reason that undefined == false returns false. Although it does not directly say why this is so, the most important part in answering this question lies in this sentence:

The comparison x == y, where x and y are values, produces true or false.

If we look up the definition for null, we find something like this:

NULL or nil means "no value" or "not applicable".

In Javascript, undefined is treated the same way. It is without any value. However, false does have a value. It is telling us that something is not so. Whereas undefined and null are not supposed to be giving any value to us. Likewise, there is nothing it can convert to for its abstract equality comparison therefore the result would always be false. It is also why null == undefined returns true (They are both without any value). It should be noted though that null === undefined returns false because of their different types. (Use typeof(null) and typeof(undefined) in a console to check it out)

What I'm curious of though, is that comparing NaN with anything at all will always return false. Even when comparing it to itself. [ NaN == NaN returns false]

Also, another odd piece of information: [ typeof NaN returns "number"]

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Strict Equality

If possible, you should avoid using the == operator to compare two values. Instead use === to truly see if two values are equal to each other. == gives the illusion that two values really are exactly equal when they may not be by using coercion. Examples:

5 == "5" is true

5 === "5" is false

"" == false is true

"" === false is false

0 == false is true

0 === false is false

Answer 2

This is so because it is so. :)

Read the ECMA standards here: https://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

Answer 3

So undefined really means undefined. Not False, not True, not 0, not empty string. So when you compare undefined to anything , the result is always false, it is not equal to that.

Answer 4

From the incomparable MDN, sponsored by the company of JavaScript's creator.

JavaScript provides three different value-comparison operations:

• strict equality (or "triple equals" or "identity") using ===,
• loose equality ("double equals") using ==,
• and Object.is (new in ECMAScript > 6).

The choice of which operation to use depends on what sort of comparison you are looking to perform.

Briefly, double equals will perform a type conversion when comparing two things; triple equals will do the same comparison without type conversion (by simply always returning false if the types differ); and Object.is will behave the same way as triple equals, but with special handling for NaN and -0 and +0 so that the last two are not said to be the same, while Object.is(NaN, NaN) will be true. (Comparing NaN with NaN ordinarily—ie, using either double equals or triple equals—evaluates to false, because IEEE 754 says so.) Do note that the distinction between these all have to do with their handling of primitives; none of them compares whether the parameters are conceptually similar in structure. For any non-primitive objects x and y which have the same structure but are distinct objects themselves, all of the above forms will evaluate to false.

For a visual overview of the whole picture of equality in JavaScript: https://dorey.github.io/JavaScript-Equality-Table/

The truth is, this seemingly "bad" aspect of JavaScript is a source of power when you understand how it works.

Answer 5

Undefined can't = false because it has not been defined so it's not known whether it does or not

In english it would be like saying Anonymous = Fred or Anonymous != Fred.

It might be Fred, it might be Bill, but at the moment we haven't a clue.

Similar rule for Null. It's slightly painful to get your head round at first, but it's a very valuable rule, without it all sorts of ambiguities can creep into your designs.

For instance how to tell the difference between something that hasn't been entered (null) or being blank.

Answer 6

According to the specification which was mentioned above:

If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

Number(undefined) = NaN;

false == NaN // false

Moreover if we change order false == undefined

If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

Number(false) = 0;
0 == undefined

There is no rule for this case, so work the default behavior:

Return false.

Answer 7

You question is half, as we compare undefined/ null to any other types. we will have false return. There is no coercion happening, even we are using == operator.

Answer 8

Undefined is not the same thing as false, false is a boolean object (which has a value of 0 therefore it is indeed defined).

An example:

var my_var;
var defined = (my_var === undefined)
alert(defined);  //prints true.  It is true that my_var is undefined

my_var = 22;
defined = (my_var === undefined)
alert(defined);  //prints false.  my_var is now defined

defined = (false === undefined)
alert(defined);  //prints false, false is defined

defined = (true === undefined)
alert(defined);  //prints false, true is defined