Javascript TypeError ____ is not a function

问题:

I am currently using the Chrome console to do some debugging for a Greasemonkey script.

From the console I run var opp = document.querySelectorAll('a[class="F-reset"]'); and it works as expected.

But if I try to remove the first element with opp.splice(0,1) I get the following error

Uncaught TypeError: opp.splice is not a function     at <anonymous>:2:5     at Object.InjectedScript._evaluateOn (<anonymous>:905:140)     at Object.InjectedScript._evaluateAndWrap (<anonymous>:838:34)     at Object.InjectedScript.evaluate (<anonymous>:694:21) 

If I try to run opp[0].indexOf("a"), i get the same thing.

How would I fix this?

回答1:

Yes, this is because the result of querySelectorAll is a node list, not an array. You can apply the slice method of Array to a node list, however:

Array.prototype.slice.call(op, 0, 1); 

This works more or less as expected, because a NodeList "quacks" in just the way slice expects, i.e. it contains elements indexed sequentially. However, you should be wary of using this in general; it is much safer to simply iterate over the NodeList.

回答2:

querySelectorAll returns a NodeList. This is similar to an array (it has a .length property and you can index it with []), but it's not actually an array, and doesn't have most of the array methods. If you want to use array methods on an array-like object, you have to call the method explicitly:

Array.prototype.splice.call(opp, 0, 1); 

or:

[].splice.call(opp, 0, 1); 

However, another difference between arrays and NodeLists is that you can't modify NodeList in place, which .splice tries to do; you can only read them like arrays. You should just use .slice() to extract the parts you want. Or convert the NodeList to an array first, and then operate on that. See

Fastest way to convert JavaScript NodeList to Array?

回答3:

querySelector/All returns a NodeList not an array, so those functions are not available.

You can use call to use those array methods though

[].splice.call(opp,0,1); 

The first argument is the execution context that the function will use, all other arguments are the arguments that will be passed to the function

Function call reference

回答4:

First,

• splice is a method of Arrays, inherited through Array.prototype, although it is intentionally generic so can be called on other Arraylike objects
• querySelectorAll returns a non-live NodeList, this is not an Array and does not share any inheritance with Array, meaning you can't simply access Array methods through it
• A function can be invoked with a custom this via call or apply
• splice needs to be able to assign on it's this, which will fail for a NodeList as you will get the following TypeError: Cannot set property length of #<NodeList> which has only a getter
• Other intentionally generic Array methods which only read from this will work on a NodeList, e.g. slice, map, indexOf, forEach, filter, some, every, etc..

Now we are in a position to do something,

1. Convert the NodeList to an Array and store this reference, i.e. with Array.prototype.slice
2. Perform your splice on this object instead

So,

var opp = document.querySelectorAll('a[class="F-reset"]'); // NodeList oop = Array.prototype.slice.call(oop); // Array // ... oop.splice(0, 1); 

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