学习JavaScript时有趣的结果。 但是我不知道为什么

Question

I wrote the following code and got no output. Here is the code

var a1 = undefined;
var a2 = 5;
if(a1 > a2)
    alert(1);
if(a1 < a2)
    alert(2);
if(a1 >= a2)
    alert(3);
if(a1 <= a2)
    alert(4);

There was no alert box that came up which means that the if statements resulted in false. Can I know the reason?

Answer 1

JavaScript tries to casts the value of x to number using ToPrimitive (@RobG). Since x is undefined, this will return NaN which compares false to any value. So it will always return false .

Answer 2

Where expressions using relational operators are involved, the Abstract Relational Comparison Algorithm is used to evaluate the operands, then the result is converted to true or false .

In the comparison algorithm, step 3a converts undefined to NaN . In step 3c it says that comparing NaN to anything returns undefined .

In the steps for say the Less-than Operator , a result of undefined is converted to false.

So for the if statements in the OP, every test returns false since they are all relational operators that all use the abstract relational comparison algorithm and one of the operands is undefined .

Answer 3

As @jAndy mentioned in the comments, You can't consider undefined keyword as a value/Instance . undefined is not a numeric type. Rather, it's a special value of the property undefined of the Global Object

That's why the Expression if(undefined {operator} {operand value}) evaluates to false. But consider the code if(undefined == undefined) returns true

Hope this Clears the ambiguity!