How to handle multiple window.onload events at the same time when referencing other js_javascript skills

Sometimes when referencing other js, its js uses the window.onload event. In this case, the onload event of the imported page may not be executed. How can I run both? In addition to the method of writing the two together, there are other methods

if(window.onload!=null){ 
eval("theOldFun="+window.onload.toString()); 
window.onload=function(){theOldFun();addReadResource();}; 
}

The role of the eval() function:

Its function is to parse the corresponding string into JS code and run it
For example, you now want to run a variable method

function name1(){……} 
function name2(){……} 
var m="name1"; 
eval(m+'()');//运行name1(); 
m='name2'; 
eval(m+'()');//运行name2();