What does a comma do in JavaScript expressions?

问题

If I use:

1.09 * 1; // returns \"1.09\"

But if I use:

1,09 * 1; // returns \"9\"

I know that 1,09 isn\'t a number.

What does the comma do in the last piece of code?

More Examples

if (0,9) alert(\"ok\"); // alert
if (9,0) alert(\"ok\"); // don\'t alert

alert(1); alert(2); alert(3); // 3 alerts
alert(1), alert(2), alert(3); // 3 alerts too

alert(\"2\",
    foo = function (param) {
        alert(param)
    },
    foo(\'1\')
)
foo(\'3\'); // alerts 1, 2 and 3

回答1:

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

Source: https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special_Operators/Comma_Operator

For example, the expression 1,2,3,4,5 evaluates to 5. Obviously the comma operator is useful only for operations with side-effects.

console.log(1,2,3,4,5);
console.log((1,2,3,4,5));

回答2:

Some more to consider:

console.log((0, 9));
console.log((9, 0));
console.log(("foo", "bar"));

回答3:

Have a look here - the comma stands for multiple expressions / statements. For example in your code you could use a line like this:

var a=0, b=0, c=0;

This would declare all three variables without writing:

var a=0;
var b=0;
var c=0;

Hope that helps.

回答4:

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

https://stackoverflow.com/a/3561056/5934465

It should be like this!

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-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator

回答5:

Adding/modifying properties to an object and returning it in the same line is a possible use-case:

console.log(
  ((x) => (o = {biggerCond: r => r >= x},
           o.r5 = Array.from(window.crypto.getRandomValues(new Uint16Array(5))),
           o.isAnyBigger = o.r5.some(o.biggerCond),
           o.bigger = o.isAnyBigger ? o.r5.filter(o.biggerCond) : [x], o )
  )(5e4)
);
// Example
// {
//   bigger: [58414, 56500, 63397],
//   isAnyBigger: true,
//   isBiggerCond: r => r >= x,
//   r5: [58414, 12015, 56500, 63397, 43861]
// }

The above anonymous function returns an object with random values bigger than the input value or, if there's none, with the input value itself in an array in contained in the bigger property.

It is still syntactic sugar (like arrow functions), but it does shorten the number of lines... I wonder if some JS minifiers detect and adjust the code in a similar way automatically. Run it in your console:

((x)=>(o={biggerCond:r=>r>=x},o.r5=Array.from(window.crypto.getRandomValues(new Uint16Array(5))),o.isAnyBigger=o.r5.some(o.biggerCond),o.bigger=o.isAnyBigger?o.r5.filter(o.biggerCond):[x],o))(5e4)

来源：https://stackoverflow.com/questions/3561043/what-does-a-comma-do-in-javascript-expressions

标签

javascript

comma-operator