Cannot read property 'style' of undefined — Uncaught Type Error

问题

I would like to change the color, fontsize and font weight of the text in a span element of the html page.

I am using the following code:

if(window.location.href.indexOf("test") > -1){
    var search_span = document.getElementsByClassName("securitySearchQuery");
    search_span[0].style.color = "blue";
    search_span[0].style.fontWeight = "bold";
    search_span[0].style.fontSize = "40px";
}

Following is the code for my html page

<h1 class="keyword-title">Search results for<span class="securitySearchQuery"> "hi".</span></h1>

I thought of getting elements by id but unfortunately they only classes and no ids. I do not have access to change the html code but just to add js code to website externally.

I have tried to look into other stackoverflow posts but could find the solution. Am new to js and css,Please let me know where am going wrong.

回答1:

Add your <script> to the bottom of your <body>, or add an event listener for DOMContentLoaded following this StackOverflow question.

If that script executes in the <head> section of the code, document.getElementsByClassName(...) will return an empty array because the DOM is not loaded yet.

You're getting the Type Error because you're referencing search_span[0], but search_span[0] is undefined.

This works when you execute it in Dev Tools because the DOM is already loaded.

回答2:

It's currently working, I've just changed the operator > in order to work in the snippet, take a look:

window.onload = function() {

  if (window.location.href.indexOf("test") <= -1) {
    var search_span = document.getElementsByClassName("securitySearchQuery");
    search_span[0].style.color = "blue";
    search_span[0].style.fontWeight = "bold";
    search_span[0].style.fontSize = "40px";

  }

}

<h1 class="keyword-title">Search results for<span class="securitySearchQuery"> "hi".</span></h1>

来源：https://stackoverflow.com/questions/42368773/cannot-read-property-style-of-undefined-uncaught-type-error

标签

javascript

html

css

dom

javascript-objects