Why can't I use Array.forEach on a collection

问题:

This question already has answers here:

How to loop through all the elements returned from getElementsByTagName [duplicate] (10 answers)

Closed 3 years ago.

I'm building an app with Babel/ES6. I want to disable all form elements for a view-only version of it, so I did this:

let form = document.getElementById('application-form')
let elements = form.elements

I expected to be able to do this, instead of using a regular old for loop (which did work):

elements.forEach((el) => {
    el.disabled = true
})

but I got TypeError: elements.forEach is not a function

The strange thing is if I console.log(elements) in the Chrome dev console it exactly like an array with a bunch of input objects. It doesn't display with the Object notation for objects, and all of the keys are integers. I'm thinking it's some sort of pseudo array, but I wouldn't even know how to find that out.

EDIT: short answer it's not an array it's an HTMLCollection. see Why doesn't nodelist have forEach?

*UPDATE*

Per this answer, nodelist now has the forEach method!

回答1:

You can. You just can't use it like that, because there is no forEach property on the HTMLFormControlsCollection that form.elements gives you (which isn't an array).

Here's how you can use it anyway:

Array.prototype.forEach.call(form.elements, function(element) {
    // ...
});

---

Or you may be able to use spread notation:

[...elements].forEach(function(element) {
    // ...
});

...but note that it relies on the HTMLFormControlsCollection implementation in the browser being iterable.

---

Or alternately Array.from (you'd need a polyfill for it, but you tagged ES2015, so...):

Array.from(elements).forEach(function(element) {
    // ...
});

For details, see the "array-like" part of my answer here.

回答2:

You cannot use forEach on HMTLCollection. forEach can only be used on `array.

Alternatives are, use lodash and do _.toArray(), which will convert the HTMLColelction to an array. After this, you can do normal array operations over it. Or, use ES6 spread and do forEach()

Like this,

var a = document.getElementsByTagName('div')
[...a].forEach()

回答3:

form.elements, document.getElementsByTagName, document.getElementsByClassName and document.querySelectorAll return a node list.

A node list is essentially an object that doesn't have any methods like an array.

If you wish to use the node list as an array you can use Array.from(NodeList) or the oldschool way of Array.prototype.slice.call(NodeList)

// es6
const thingsNodeList = document.querySelectorAll('.thing')
const thingsArray = Array.from(thingsNodeList)
thingsArray.forEach(thing => console.log(thing.innerHTML))

// es5
var oldThingsNodeList = document.getElementsByClassName('thing')
var oldThingsArray = Array.prototype.slice.call(oldThingsNodeList)
thingsArray.forEach(function(thing){ 
  console.log(thing.innerHTML) 
})

<div class="thing">one</div>
<div class="thing">two</div>
<div class="thing">three</div>
<div class="thing">four</div>