Uncaught TypeError: Cannot read property 'length' of undefined.

这是我如何去做.我检查DOM的准备情况,然后调用该功能.

function walkmydog() {
    //when the user starts entering                                                                                                                                                
    if(document.getElementById('WallSearch').value.length == 0) {
        alert("nothing");
    }
}

if (document.addEventListener) {
    document.addEventListener("DOMContentLoaded",walkmydog,false);
}