为什么console.log函数返回undefined？

Question

 function stringifyObj(parmObj){ s=""; Object.getOwnPropertyNames(parmObj).forEach ( function (val, idx, array) { s+=val + ' -> ' + parmObj[val]+"\\n"; } ) return s; } var arrayOfObjects = [ { name: 'Edward', value: 21 }, { name: 'Sharpe', value: 37 } ]; console.log( arrayOfObjects.forEach(function(parmArrItem) { const p=stringifyObj(parmArrItem); console.log(p); } )); 

In the code below, 2 objects are displayed fine, but after that I get undefined displayed at the end of the run. Where does the undefined come from? Thanks.

Answer 1

arrayOfObjects.forEach returns nothing.

So, when you use console.log() for a void function that's you received undefined .

forEach method only executes a callback provided function for every item from an array.

With the other words, the console prints the result of evaluating an expression .

console.log() is undefined since your function or expression not explicitly return something.

 function stringifyObj(parmObj){ s=""; Object.getOwnPropertyNames(parmObj).forEach ( function (val, idx, array) { s+=val + ' -> ' + parmObj[val]+"\\n"; } ) return s; } var arrayOfObjects = [ { name: 'Edward', value: 21 }, { name: 'Sharpe', value: 37 } ]; arrayOfObjects.forEach(function(parmArrItem) { const p=stringifyObj(parmArrItem); console.log(p); } );