Much simpler! Sort of. The code is shorter, but also requires greater familiarity with Vue instance properties. In this case, you have to know that when you pass children without a v-slot directive into a component, like the Hello world! Take this HTML for example:. Every element is a node. Every piece of text is a node.
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Even comments are nodes! A node is just a piece of the page. And as in a family tree, each node can have children i. Updating all these nodes efficiently can be difficult, but thankfully, you never have to do it manually.
And in both cases, Vue automatically keeps the page updated, even when blogTitle changes. Taking a closer look at this line:. What is createElement actually returning? It could perhaps more accurately be named createNodeDescriptionas it contains information describing to Vue what kind of node it should render on the page, including descriptions of any child nodes.
Notice that the key names within this object match the names that we specify within the ref attribute, and the values are the DOM elements. In this case, we can see that the key is myButton and that the value is a native button element which has nothing to do with Vue. As an example, I will change the text of the button when clicking it.
There is a potential problem that you should be aware of, which I will show you now.
Since I have already assigned the Vue instance to a variable, I can just go ahead and use it. What I want to do, is to change the text of the element. Since it will initially contain the value of the message data property, I will just apply a timeout so that we can see what happens. As you can see, the change that we made to the DOM is overwritten when updating the data property. The reason for this, is that when accessing the DOM elements and manipulating them directly, we are essentially skipping the virtual DOM as we discussed in a previous post.
So Vue is still controlling the h1 element because it holds a copy of the template, and when Vue reacts to changes in the data property, it updates the virtual DOM and thereafter the DOM itself. Therefore you should be careful with applying changes to the DOM directly like this, as any changes you apply may be overwritten if you are not careful. While you should be cautious with changing the DOM when using references, it is safer to do read-only operations such as reading values from the DOM.
As the last thing, I want to show you how you can also use the ref attribute on elements with the v-for directive. I will just add an unordered list consisting of the numbers from one to ten, which I will output using the v-for directive. A numbers property has been added to the object as we would expect, but notice the type of the value. When using the ref attribute together with the v-for directive, Vue collects the DOM elements for all of the iterations of the loop and places them within an array.
In this case, this gives us an array of ten li DOM elements, because our loop iterates ten times. Each of these elements can then be used exactly as we saw before.
Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I am trying to get the canvas element which is inside a template of a component, found great documentations for vuejs1 but not for vuejs2 where "ref" is the only way to get the element.
I am getting the object though, but when I try to access the variable it is undefined. The created is fired before the template is processed. I also ran into this error. The way i fixed this issue was by getting the refs in the updated hook. See my example below. In the updated hook I check if any refs are found. If not then nothing will happen. Then when there are products found, the script will continue. Next time Vue will come into the updated hook again, your script won't be triggerd again because now the length of the products array is bigger than one if any refs could be found of course.
Learn more. Vuejs can't access refs from component Ask Question. Asked 3 years, 4 months ago.
Active 7 months ago. Viewed 44k times. Paolo Code Tree Code Tree 1, 1 1 gold badge 18 18 silver badges 26 26 bronze badges. Give the canvas an ID and use this: this. Active Oldest Votes. Mihai Vilcu Mihai Vilcu 1, 14 14 silver badges 22 22 bronze badges. That's right. Unfortunately, it's still an issue if your ref is on a v-for element. The DOM isn't acctualy eable to access this ref when mounted hook is fired. Any suggestion about this? BrownBe, there is still no answers. BrownBe, check my answer, maybe it can solve your problems.Check it out!
Don't worry, you're not alone — I've run into this issue countless times, and I'll show you just how to solve it. The likely cause of this is that you're mixing up your usage of regular functions and arrow functions. My guess is that you're using an arrow function. If you replace the arrow function with a regular function it will probably fix the issue for you.
In a regular function like this one, this will refer to the "owner" of the function. Since we're defining it on the Vue component, this refers to your Vue component. I'll explain how this scoping works in more detail later on. Arrow functions can be even shorter and quicker to write, and have gained lots of popularity recently because of this.
However, they aren't too different when defining a method on an object like we are doing when writing Vue components. The real differences start to come in to play when dealing with how this works. In an arrow function, this does not refer to the owner of the function. An arrow function uses what is called lexical scoping. We'll get into this more in a bit, but it basically means that the arrow function takes this from it's context.
If you try to access this from inside of an arrow function that's on a Vue component, you'll get an error because this doesn't exist! So in short, try to avoid using arrow functions on Vue components. It will save you a lot of headaches and confusion.
There are times when it's nice to use a short arrow function. But this only works if you aren't referencing this :. Anonymous functions are great for when you just need to create a quick function and don't need to call it from anywhere else. They're called anonymous because they aren't given a name, and aren't tied to a variable.
As you can see from the examples, most of the time when people create anonymous functions they use arrow functions. We typically use arrow functions for several reasons:. But wait, didn't we just figure out that arrow functions don't work when we try to access this? When we use arrow functions inside a regular or shorthand functionthe regular function sets this to be our Vue component, and the arrow function uses that this say that 5 times fast!
Our filter can access this. This method, because it is a regular function and not an arrow functionsets it's own context to be the Vue instance. Let's expand further on how you would this to fetch data using axios or fetch. If you're fetching async data using fetch or axiosyou're also using promises. Promises love anonymous arrow functions, and they also make working with this a lot easier. If you're fetching some data and want to set it on your component, this is how you'd do that properly:.
Notice how we're using a regular function as the method on the Vue component, and then using anonymous arrow functions inside of the promise:. Inside of the scope for fetchDatawe have that this is set to our Vue component because it is a regular function. Since arrow functions use the outer scope as their own scope, the arrow functions also set this to be our Vue component.Read that first if you are new to components.
It replaces the slot and slot-scope attributes, which are now deprecated, but have not been removed and are still documented here. The rationale for introducing the new syntax is described in this RFC. Slots can contain any template code, including HTML:. That slot has access to the same instance properties i.
For example, trying to access url would not work:. Everything in the parent template is compiled in parent scope; everything in the child template is compiled in the child scope. Updated in 2.
How to fix "this is undefined" in Vue
See here for the deprecated syntax using the slot attribute. See here for the deprecated syntax using the slot-scope attribute. This allows us to use v-slot directly on the component:. This can be shortened even further. Just as non-specified content is assumed to be for the default slot, v-slot without an argument is assumed to refer to the default slot:.
So in supported environments single-file components or modern browsersyou can also use ES destructuring to pull out specific slot props, like so:. This can make the template much cleaner, especially when the slot provides many props.
It also opens other possibilities, such as renaming props, e. Dynamic directive arguments also work on v-slotallowing the definition of dynamic slot names:. Similar to v-on and v-bindv-slot also has a shorthand, replacing everything before the argument v-slot: with the special symbol. For example, v-slot:header can be rewritten as header :. However, just as with other directives, the shorthand is only available when an argument is provided.
That means the following syntax is invalid:. Slot props allow us to turn slots into reusable templates that can render different content based on input props. This is most useful when you are designing a reusable component that encapsulates data logic while allowing the consuming parent component to customize part of its layout.
Instead of hard-coding the content for each todo, we can let the parent component take control by making every todo a slot, then binding todo as a slot prop:. However, even this barely scratches the surface of what scoped slots are capable of. For real-life, powerful examples of scoped slot usage, we recommend browsing libraries such as Vue Virtual ScrollerVue Promisedand Portal Vue.
The v-slot directive was introduced in Vue 2. The full rationale for introducing v-slot is described in this RFC. The slot and slot-scope attributes will continue to be supported in all future 2. Deprecated in 2. See here for the new, recommended syntax.For example, given the example with jQuery above, we can make a call to noConflict to restore the variable.
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Already on GitHub? Sign in to your account. Sometimes, when some of each refresh page, mounted lifecycle of component can't get this. I know mounted doesn't guarantee completed about render. But It is caused child component but I am not associate with child component. I got braking point on mounted code like below capture. The location that.
Can you provider all code in your case? That's not the case. HyunmoAhn Your problem seems unrelated. So, I think its type should include undefined. It's always an object. It just may be an empty object sometimes, so why this.
LinusBorgyea, its right, I misstated it with this. Can you reopen the issuse with rename, I want to request pr to fix it.
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From what I understand the OP is referring to typescript only, because the type doesn't get automatically picked up by the compiler when he does. I recommend you check Type Guards and Differentiating Types. In regards to runtime error, you should always check if the ref is not undefined and if is an instance or an array. I promise add loading when fetching data and vccCredit. Skip to content. Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.
Sign up. New issue. Jump to bottom. Copy link Quote reply. Version 2. What is expected? Throw error in compiler-time. What is actually happening? Throw error in runtime. This comment has been minimized. Sign in to view. I have same Issue. And, It is occurred randomly! Please Tell whether it is bug or not.