小于等于运算符：使用NaN

Question

When we use The Less-than-or-equal Operator this is work under the hood with The Abstract Relational Comparison Algorithm. For example.

a <= b;

Convert to JavaScript like this

!(b < a)

And EcmaScript Spesification says ( http://www.ecma-international.org/ecma-262/5.1/#sec-11.8.5 ) which indicates that at least one operand is NaN less than return undefined And this is meaning

var a = 1;
var b = "asd"
a < b // b.toNumber() => NaN and this is operation return undefined (false)

If we use like this

var a = 1;
var b = "asd"
a <= b // this convert to  !(b < a) and (b<a) return undefined
// and !(undefined) must be true

But EcmaScript spesification says this is return false. This is interesting for me what is reason this?

Answer 1

While <= does use the Abstract Relational Comparison Algorithm, a <= b isn't equivalent to !(b < a) . It is equivalent to b < a !== false ? false : true b < a !== false ? false : true (where < represents the Abstract Relational Comparison Algorithm, not the JavaScript < operator which can never evaluate to undefined ), which behaves the same as !(b < a) when b < a is truthy or exactly false , but does not behave the same when b < a is falsey in general. If b < a evaluates to undefined , the whole expression will evaluate to false .

This is defined in the spec at step 6 here: https://www.ecma-international.org/ecma-262/5.1/#sec-11.8.3

5. Let r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false .
6. If r is true or undefined , return false . Otherwise, return true .

The Abstract Relational Comparison Algorithm can only evaluate to true , false , or undefined ; so the "Otherwise" in step 6 can only apply when r is false , making a <= b equivalent to b < a !== false ? false : true b < a !== false ? false : true (again, where < represents the Abstract Relational Comparison Algorithm).