Why does javascript accept commas in if statements?

Question

I stumbled across some javascript syntax that seemed like it should produce a parse error of some kind but doesn't: if (true, true) {console.log('splendid')} else {console.log('horrid')} // splendid if (true, false) {console.log('splendid')} else {console.log('horrid')} // horrid It seems only the last expression affects the logic, though all expressions are executed: if (console.log('super'), true) {console.log('splendid')} // super splendid Anyone know why that is valid javascript syntax? Is there any practical use for it?

Answer 1

 In a conditional statement the comma is treated like a sequence so there is not parse error that happens. So what really happens is that the first operand in this case they're both true, but the first "true" gets evaluated and then the results gets discarded. It then evaluates the second true value and returns the result back.

Answer 2

"Valid use" is in the eye of the beholder, but there are a few clever things someone can use it for. I'm not sure if one would consider it best practices. One example

[1,2,3,4].reduce((a, c) => (a[c] = true, a), {});

Notice how the comma is used there so that we can inline that method as opposed to writing out the whole thing?

Another one is that people use it to do things to multiple values in a for loop

for (let i = 0, j = 0; i < 50; i++, j += 10) {

// do stuff

}

There are likely others that I am not aware of, but in my experience its generally used to save space and time.