C. Lee - 1 year ago 100
Javascript Question

# Why `null >= 0 && null <= 0` but not `null == 0`?

I had to write a routine that increments the value of a variable by 1 if its type is

`number`
and assigns 0 to the variable if not, where the variable is initially
`null`
or
`undefined`
.

The first implementation was
`v >= 0 ? v += 1 : v = 0`
because I thought anything not a number would make an arithmetic expression false, but it was wrong since
`null >= 0`
is evaluated to true. Then I learned
`null`
behaves like 0 and the following expressions are all evaluated to true.

• `null >= 0 && null <= 0`

• `!(null < 0 || null > 0)`

• `null + 1 === 1`

• `1 / null === Infinity`

• `Math.pow(42, null) === 1`

Of course,
`null`
is not 0.
`null == 0`
is evaluated to false. This makes the seemingly tautological expression
`(v >= 0 && v <= 0) === (v == 0)`
false.

Why is
`null`
like 0, although it is not actually 0?

Your real question seem to be:

Why:

``````null >= 0; // true
``````

But:

``````null == 0; // false
``````

What really happens is that the Greater-than-or-equal Operator (`>=`), performs type coercion (`ToPrimitive`), with a hint type of `Number`, actually all the relational operators have this behavior.

`null` is treated in a special way by the Equals Operator (`==`). In a brief, it only coerces to `undefined`:

``````null == null; // true
null == undefined; // true
``````

Value such as `false`, `''`, `'0'`, and `[]` are subject to numeric type coercion, all of them coerce to zero.

You can see the inner details of this process in the The Abstract Equality Comparison Algorithm and The Abstract Relational Comparison Algorithm.

In Summary:

• Relational Comparison: if both values are not type String, `ToNumber` is called on both. This is the same as adding a `+` in front, which for null coerces to `0`.

• Equality Comparison: only calls `ToNumber` on Strings, Numbers, and Booleans.

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