Your code is working, check this fiddle DEMO
EDIT
Remove the script
tag from your jsfiddle
leave only the code.
function hidepre() {
var elems = document.getElementsByClassName("prewrap");
for (var i = 0; i < elems.length; i++) {
if (elems[i].style.display === "none") {
elems[i].style.display = "block";
} else {
elems[i].style.display = "none";
}
}
}
var imgs = document.getElementsByTagName("img");
for (var i = 0; i < imgs.length; i++) {
imgs[i].onclick = hidepre;
}
Also in your post this line should be corrected ->
var imgs = document.getElementsByTagName("img");
for (var i = 0; i < imgs.length; i++) {
imgs[i].onclick = hidepre; //this line - here should only be the name of the function without quotes and parentheses
}