Vue cannot obtain the id attribute because getElementById is used in the "created()" hook function, and Vue has not yet completed mounting; the solution is to change "created() {let serachBox = document. getElementById('searchBox');...}" code can be migrated to the "mounted()" hook function.
The operating environment of this tutorial: Windows 10 system, vue3 version, DELL G3 computer
What should I do if vue cannot obtain the id attribute?
The result of using getElementById in Vue is that the returned element is null?
- First of all, I used such a requirement in the project to dynamically calculate the height of the main area based on the client screen size.
When I get this requirement, my first idea is to get the size of the fixed element through element.getBoundingClientRect, and then get the size of the fixed element through document.body.offsetHeight The height of the view area, and finally the size of the main area is dynamically calculated.
Use getElementById in created()
After confirming the idea, I started coding, so I wrote the following code in created:
//此處省去無關(guān)代碼
created()?{
let?serachBox?=?document.getElementById('searchBox');
let?searchRect?=?serachBox?.getBoundingClientRect();
console.log('rect',searchRect?);
let?dffsetHeight?=?document.body.offsetHeight;
this.tHeight?=?(?dffsetHeight?-?searchRect?.bottom?-?100?);},
As a result, the console reported an error directly. The error was as follows: 
According to the error description, I printed the searchBox element upwards. At this time, the console printed the result as null. , this is a bit interesting.
Using getElementById in mounted()
Looking carefully, it turns out that I used getElementById in the created() hook function. At this time Vue has not completed mounting, so it cannot obtain the Dom element through getElementById, so the console prints null. After finding the reason, I migrated the above code to the mounted() hook function, and the console printed the correct result. 
Although the problem was found, the rendering result of the page could not meet my needs. We have to continue to find a way.
Created() is combined with this.$nextTick() to achieve business requirements
Finally created() is combined with this.$nextTick() to achieve business requirements The final code is as follows:
created()?{
this.$nextTick(function?()?{
let?serachBox?=?document.getElementById('searchBox');
let?searchRect?=?serachBox?.getBoundingClientRect();
console.log('rect',searchRect?);
let?dffsetHeight?=?document.body.offsetHeight;
this.tHeight?=?(?dffsetHeight?-?searchRect?.bottom?-?100?);
?})},
Summary
This bug is mainly caused by two aspects.
First, I habitually handle business logic in the created hook function
The second reason is that I am not familiar enough with the life cycle of Vue, and I made a mistake. confused.
Recommended learning: "vue video tutorial"
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