Introduction and usage examples of comma operators in JavaScript

There is an js interview question. The question is: what is the result of the execution of the following code and why?

var i, j, k;

for (i=0, j=0; i<10, j<6; i++, j++) {

  k = i+j;

}

document.write(k);

The answer is to display 10, which focuses on the comma operator JavaScript.

Here is MDN's definition of the comma operator:

The comma operator evaluates two operands (from left to right) and returns the value of the second operand.

According to this definition, you can extend 1:

The comma operator evaluates two or more operands from left to right and returns the value of the last operand.

You can feel 1 in the following code:

alert((0, 9));

alert((9, 0));
if (0,9) alert("ok");

if (9,0) alert("ok");

What role does the comma operator play in actual code?

1. Exchange variables without the third variable

var a = "a", b = "b";
// methods 1

a = [b][b = a, 0];
// methods 2

a = [b, b = a][0];

2. Simplify your code

if(x){

  foo();

  return bar();

}

else{

  return 1;

}

It can be abbreviated as:

return x ? (foo(), bar()) : 1;