Question

console.log("20">10);  //true 
console.log("20a">"10");  //true 
console.log("20a">10);  //false

I want to know why the last one turns false.
And “20a” transforms to what before comparing.

Answer 1

From the MDN page on comparison operators:

For relational abstract comparisons (e.g. <=), the operands are first converted to primitives, then the same Type, before comparison.

console.log("20">10);  //true 

This converts "20" to a number 20 and compares it. Since 20 is greater than 10, it is true.

console.log("20a">"10");  //true 

This compares the two strings. Since "20a" is greater (alphabetically) than "10", it is true.

console.log("20a">10);  //false

This converts "20a" to a number. The result is NaN (do +"20a" to see this in action). NaN is not greater than any number, so it returns false.