dom querySelector

element = document.querySelector(selectors);

Returns the first element within the document (using depth-first pre-order traversal of the document's nodes) that matches the specified group of selectors.

• element is an element object.
• selectors is a string containing one or more CSS selectors separated by commas.

In this example, the first element in the document with the class "myclass" is returned:
varel = document.querySelector(".myclass");

Returns null if no matches are found; otherwise, it returns the first matching element.

 

还有一个相对应的方法：

elementList = document.querySelectorAll(selectors);
返回所有匹配的元素，放回结果是一个NodeList。 elementList is a non-live NodeList of element objects.

 

Along the lines of other frameworks such as jQuery or Prototype, shortening the "querySelector" name can be convenient.

function $ (selector, el) {
     if (!el) {el = document;}
     return el.querySelector(selector);
}
function $$ (selector, el) {
     if (!el) {el = document;}
     return el.querySelectorAll(selector);
     // Note: the returned object is a NodeList.
     // If you'd like to convert it to a Array for convenience, use this instead:
     // return Array.prototype.slice.call(el.querySelectorAll(selector));
}
alert($('#myID').id);

Example:

 

<h1>Test!</h1>

<script>

HTMLDocument.prototype.$ = function (selector) {

    return this.querySelector(selector);

};

alert(document.$('h1')); // [object HTMLHeadingElement]

</script>

现在所有的元素都有$方法了。

 

via：https://developer.mozilla.org/en-US/docs/Code_snippets/QuerySelector