Expand an iterable element or non-iterable element into an array without checking element .length

问题

Given html

<div></div>
<div></div>

calling document.querySelector("div") returns the first div element, where .length is not a property of the return value.

Calling document.querySelectorAll() returns a NodeList having a .length property.

The difference between the two return values of .querySelector() and .querySelectorAll() is that the former is not an iterable; and error will be thrown when attempting to use the spread element to expand the element into an array.

In the following examples consider that either div or divs is a parameter received within the body of a functions call. Thus, as far as can gather, it is not possible to determine if the variable was defined as a result of Element.querySelector(), Element.querySelectorAll(), document.querySelector() or document.querySelectorAll(); further the difference between .querySelector() and .querySelectorAll() can only be checked using .length.

var div = document.querySelector("div");
for (let el of div) {
  console.log(".querySelector():", el)
}

<div></div>
<div></div>

logs

Uncaught TypeError: div[Symbol.iterator] is not a function

while

var div = document.querySelectorAll("div");
for (let el of div) {
  console.log(".querySelectorAll():", el)
}

<div></div>
<div></div>

returns expected result; that is, document.querySelectorAll("div") is expanded to fill the iterable array.

We can get the expected result at .querySelector() by setting div as an element of an Array

[div]

at for..of iterable parameter.

The closest have come to using same pattern for both or either .querySelector() or .querySelectorAll() is using callback of Array.from() and the .tagName of the variable, and spread element. Though this omits additional selectors that may have been called with .querySelector(), for example .querySelector("div.abc").

var div = document.querySelector("div");
var divs = document.querySelectorAll("div");
var elems = Array.from({length:div.length || 1}, function(_, i) {
  return [...div.parentElement.querySelectorAll(
    (div.tagName || div[0].tagName))
         ][i]
});

for (let el of elems) {
  console.log(".querySelector():", el)
}

elems = Array.from({length:divs.length || 1}, function(_, i) {
  return [...divs[0].parentElement.querySelectorAll(
    (divs.tagName || divs[0].tagName))
         ][i]
});

for (let el of elems) {
  console.log("querySelectorAll:", el)
}

<div></div>
<div></div>

This does not provide adequate accuracy for additional reasons; Element.querySelector() could have been originally passed to function, instead of document.querySelector(), similarly for .querySelectorAll(). Not sure if it is possible to retrieve the exact selector passed to.querySelector, All` without modifying the native function?

The desired pattern would accept the variable, and expand the contents of the iterable into an array if an .querySelectorAll() was used; which would treat .getElementsByTagName(), .getElementsByClassName(), .getElementsByTagName(), .getElementsByName() the same; or set the single value returned by .querySelector() as element of the array.

Note, the current working solution is

div.length ? div : [div]

which iterates div if div has a .length property, possibly an iterable, though simply have a .length property and not be an iterable; else set div as single element of an array, an iterable.

var div = document.querySelector("div");
var divs = document.querySelectorAll("div");
var elems = div.length ? div : [div];

for (let el of elems) {
  console.log(".querySelector():", el)
}

var elems = divs.length ? divs : [divs];

for (let el of elems) {
  console.log("querySelectorAll:", el)
}

<div></div>
<div></div>

Can this be achieved

• without checking the .length of the variable?
• without referencing the element three times on same line?

Can the approach of the working solution

• be improved; that is should [Symbol.iterator] of div be checked instead of .length?
• is there magic using .spread element or rest element which could allow omission of checking .length of object?
• would using a Generator, Array.prototype.reduce() or other approach change the need to check the .length or [Symbol.iterator] property of a variable before expanding the element into an array?

Or, is the above the approach the briefest possible given the difference between objects which are iterable or not iterable?

回答1:

I’d do more or less what Array.from does, but check the type of length instead of always converting it:

const itemsOrSingle = items => {
    const iteratorFn = items[Symbol.iterator]

    if (iteratorFn) {
        return Array.from(iteratorFn.call(items))
    }

    const length = items.length

    if (typeof length !== 'number') {
        return [items]
    }

    const result = []

    for (let i = 0; i < length; i++) {
        result.push(items[i])
    }

    return result
}

来源：https://stackoverflow.com/questions/40198807/expand-an-iterable-element-or-non-iterable-element-into-an-array-without-checkin