爲什麼函數返回element.innerHTML或element.innerText未定義？

Question

爲什麼function(){ return element.innerHTML }返回undefined？

var demo = document.getElementById('demo'); 
demo.innerHTML = endangeredSpecies("Europe", "Cave bear"); 

function endangeredSpecies(continent, species) { 
    var contin = document.querySelectorAll('[data-continent]'); 
    contin.forEach(function(div){ 
    if(div.getAttribute('data-continent') == continent){ 
       var children = [].slice.call(div.children); 
     children.forEach(function(child) { 
        if(child.getAttribute('data-species') == species) { 
       return child.innerText; 
       } 
      }) 
    } 
    }) 
} 

Fiddle

Answer 1

你的功能不返回任何東西，也返回裏面的forEach不返回值。它分配給一個變量和函數返回它liek

var demo = document.getElementById('demo'); 
demo.innerHTML = endangeredSpecies("Europe", "Cave bear"); 

function endangeredSpecies(continent, species) { 
    var contin = document.querySelectorAll('[data-continent]'); 
    var val; 
    contin.forEach(function(div){ 
    if(div.getAttribute('data-continent') == continent){ 
       var children = [].slice.call(div.children); 
     children.forEach(function(child) { 
        if(child.getAttribute('data-species') == species) { 
       val = child.innerText; 
       } 
      }) 
    } 
    }) 
    return val; 
} 

JSFIDDLE