在 JavaScript 中逗号运算符是做什么的？

如果我使用:

1.09 * 1; // returns "1.09"

但如果我用:

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

我知道109不是个数字。

最后一段代码中的逗号是做什么的？

更多例子

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
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最佳答案

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));

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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.

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Some more to consider:

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

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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)
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The specific syntax allows you to functionally bake bread and hand it to the customer to consume with no returns.

(new Array(3)).fill()
.map(()=>({state:"dough", bake(){this.state="baked"}, consume(){this.state="consumed"}}))


.map(bread=>(console.log(`Adding ${bread.state} to oven.`), bread.bake(), bread))
.map(bread=>(console.log(`Consuming ${bread.state} bread.`), bread.consume(), bread))
.map(bread=>console.log(`Bread is now ${bread.state}.`))

Adding dough to oven.
Adding dough to oven.
Adding dough to oven.
Consuming baked bread.
Consuming baked bread.
Consuming baked bread.
Bread is now consumed.
Bread is now consumed.
Bread is now consumed.

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