let obj = {a:'a',b:'b'};
for (let p in obj){
console.log(p);
}
Output:
a
b
undefined
未定义的属性是怎么回事？
这不是财产，这是函数返回 that 开发者工具 prints.
Example:
console.logger = function(p){
console.log(p);
return "logged"
}
var obj = {a:'a',b:'b'};
for (var p in obj){
console.logger(p);
}
a // logs the property
b // logs the property
"logged" // logs the return of logger function, undefined in case nothing's returned
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