Error in JavaScript "typeerror:xxx is null"

Today when writing a JavaScript demo, for a small problem for a long time, and the code is as follows:

var start = document.getElementById (' start ');

Start.onclick = function () {

...........

};

After this error was reported, Typeerror:start is null

Oops, after a look at Baidu, it turned out that the JavaScript code in the head, so careless. After thinking about it, it is true. The JavaScript in the head defines the document structure, but the entire document is not loaded at that time, And I didn't write the whole code in window.onload = function () {}.

It is common practice to write JavaScript code behind the body so that it does not produce this error

Error in JavaScript "typeerror:xxx is null"