for of loop querySelectorAll

问题:

Mozilla states that "for of loops will loop over NodeList objects correctly". (source: https://developer.mozilla.org/en-US/docs/Web/API/NodeList) However, this doesn't work in Chrome 43. Is this incorrect documentation or a browser bug?

The copied example code used on a page with checkboxes:

var list = document.querySelectorAll( 'input[type=checkbox]' );
for (var item of list) {
  item.checked = true;
}

回答1:

Edit: This is shipping in Chrome 51.

Jake Archibald posted a simple fix:

NodeList.prototype[Symbol.iterator] = Array.prototype[Symbol.iterator]

And for of loops.

回答2:

The docs are correct, but I wouldn't call this a bug. Rather it's a "not yet implemented feature".

There is no standard for this, and there is still active discussion on how the DOM should integrate with ES6. Notice that it is clear that querySelectorAll should return something iterable which can be used in a for of loop (as the common expectation demands), but it's not clear how that should happen (Let NodeList implement the Iterable interface? Let some Elements collection subclass Array?).

回答3:

Since I've successfully used for..of in Gecko to iterate NodeLists, it seems this is a browser bug, or at least a browser lack.

Actual working code from a userscript I currently use:

let llnk = document.querySelectorAll("div#threadlist a.threadtitle_unread");
for (let lnk of llnk) {
    //...
}

(This also uses let, but that's another story.)

回答4:

You can use Array.from

let _list = document.querySelectorAll('input[type=checkbox]');

let list = Array.from(_list);

for (let item of list) {
  //... just like an array
  item.checked = true
}

or more shortly

let list = document.querySelectorAll('input[type=checkbox]');

for (let item of Array.from(list)) {
  item.checked = true
}

Important note Array.from was introduced in Chrome 45 source.

回答5:

Try utilizing Array.prototype.entries()

var list = [].entries.call(document.querySelectorAll("input[type=checkbox]"));

for (item of list) {
  item[1].checked = true;
};

<input type="checkbox" /><input type="checkbox" />

You could also use Array.prototype.values()

var list = [].values.call(document.querySelectorAll("input[type=checkbox]"));

for (item of list) {
  item.checked = true;
};

<input type="checkbox" /><input type="checkbox" />

回答6:

Here's yet another solution for the modern age:

[...document.querySelectorAll("input[type=checkbox]")].forEach(node => {
     node.textContent = "foo";
});

This takes advantage of the spread operator which is supported in Google Chrome 46+, Firefox 16+, and Edge, and just for fun the arrow function.

回答7:

This is what I do, for a different approach

Array.prototype.forEach.call(document.querySelectorAll("input[type=checkbox]"),function(ele,idx)
{
    ele.checked = true;
}

good from IE9 and above

回答8:

Native Symbol.iterator support for NodeList was added to the WHATWG's DOM spec in 2014.

Unfortunately, Chrome 51 is the first version of Chrome to support it, and its Beta has only just been released at the time of writing this answer. Also, there's no support in any version of Internet Explorer or Edge.

To add Symbol.iterator support for NodeList in all browsers to your code, just use the following polyfill :

NodeList.prototype[Symbol.iterator] = Array.prototype[Symbol.iterator];

回答9:

I had this problem. Turns out mine was caused by calling Promise.all() with parameters instead of an array. For example:

Before: Promise.all(p1, p2)

After: Promise.all([p1, p2])

Hope this helps someone.