Why does javascript accept commas in if statements?

commas in javascript are actually pretty arcane. The coolest use I have seen is this

while(doSomething(), checkIfSomethingHappened());

the most common would be the way var is used in modern js

var foo = 1,
    bar = 2;

The comma operator chains multiple expressions together, and the result of the operation is the value of the last operand. The only real use for it is when you need multiple side effects to occur, such as assignment or function calls.

The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.

https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Operators/Special_Operators/Comma_Operator

https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Expressions_and_Operators#comma_operator