使用 ? 时的无限循环：“for”循环内的条件

Question

These are the two examples to compare the difference:

 for (var i = 0; i < 1 < 2 ? 1 : 2; i++) { console.log(i); }

It will log:

1 2 3 ... 1000 2000 3000 ...

 for (var i = 0; i < (1 < 2 ? 1 : 2); i++) { console.log(i); }

It will log:

0

My question: What is the difference between

i < 1 < 2 ? 1 : 2

and

i < (1 < 2 ? 1 : 2)

P/S: If you're using Chrome to view this question, please pressing Shift + Esc to open Chrome Task Manager to force to stop running the script in this page (If it doesn't stop after clicking Run code snippet ).

Thank you!

Answer 1

The answer is in JavaScript's operator precedence . Per that list, the < operator is higher precedence than the ?: conditional operator, and < has left-to-right associativity, so i < 1 < 2 ? 1 : 2 i < 1 < 2 ? 1 : 2 is interpreted as ((i < 1) < 2) ? 1 : 2 ((i < 1) < 2) ? 1 : 2 .

Immediately, you can see that it doesn't matter what the < conditions resolve to; the only possible results are 1 and 2 , both of which are "truthy".

For completeness, i < 1 produces true for i==0 , and false for i>0 , which when used in numeric context, has a value of 1 or 0 respectively, but since both 1 < 2 and 0 < 2 both reliably produce true , the whole expression is always evaluating to 1 .

Answer 2

Your first loop is being evaluated as this:

 for (var i = 0; (i < 1) < 2 ? 1 : 2; i++) { console.log(i); if (i > 100) break; // added to stop the infinite loop }

The expression (i < 1) will be true , or 1 , when i=0 . For values of i >= 0 , the expression (i < 1) will be false, which will be implicitly converted to 0 . For all values of i , the inequality (i < 1) < 2 will evaluate to true, so the value 1 will be the output of the ternary expression. Actually, it doesn't even matter whether (i < 1) < 2 is true or false, because the outcomes are 1 and 2, both of which are truthy. So, your loop is basically equivalent to the following:

for (var i = 0; true; i++) {
    console.log(i);
}

That is, it just boils down to an infinite loop in i .

In the demo above, I added a break statement to the loop to prevent it from crashing your browser.