Why is function <= function true, but NaN <= NaN false in ECMAscript

user_1778043 0
Viewed 60

I am trying to understand the mystery of comparing functions:

let a=function(){}
let b=function(){}
console.log(a==b)  //false
console.log(a===b) //false
console.log(a<b)   //false
console.log(a>b)   //false
console.log(a<=b)  //true !?!
console.log(a>=b)  //true !?!

console.log( (+a) <  (+b) )  //false
console.log( (+a) >  (+b) )  //false
console.log( (+a) <= (+b) )  //false
console.log( (+a) >= (+b) )  //false

In ECMAscript the <= operator is described:

RelationalExpression:RelationalExpression<=ShiftExpression

  1. Let lref be the result of evaluating RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluating ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be the result of performing Abstract Relational Comparison rval < lval with LeftFirst equal to false.
  6. ReturnIfAbrupt(r).
  7. If r is true or undefined, return false. Otherwise, return true.

The Abstract Relational Comparison seems to have special cases for strings and BigInts but I think everything else should be converting to numbers. So I would expect the second set of comparisons to be equivalent to the first and yet it's not. What am I missing?

javascript
1 Answers

The Abstract Relational Comparison calls ToPrimitive on the arguments with a hint of 'number'. That then calls OrdinaryToPrimitive again with a hint of 'number'.

The hint here is really just a hint; it is not required that the routine returns that type. The hint of 'number' just causes OrdinaryToPrimitive to try calling valueOf before toString, but it will still end up calling toString if valueOf does not return a primitive.

If you provide a valueOf implementation that returns a primitive it will be used for the comparison:

let a=function(){}
let b=function(){}

a.valueOf = () => 1;
b.valueOf = () => 0;

console.log( a <= b );
console.log( b <= a );

By default valueOf for a function returns the function itself, which is not a primitive, so toString is called instead and the functions are compared as strings.

user_7720350
Related