How does javascript implement page iframe event listening

    var iframe = document.getElementById('iframe'); 
         iframe.onload = function(){
            iframe.contentDocument.onmousemove = function () {
            // console.log("iframe"); // Get the last mouse movement event after iframe is loadedCopy the code

Gets the iframe element

iframe.onload
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The listening page is loaded

iframe.contentDocument.onmousemove
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Adding event Judgment