Question

我是箭头函数的新手，我只是在没有未定义时才尝试添加到数组但我无法弄清楚语法。我应该如何写下面的if语句？谢谢！

let carList = cars.map(car => (
  if(car.owner !== "undefined"){ // this is the condition I want.
  do something
 }
));

Answer 1

与任何其他函数一样，正文必须是大括号，而不是括号：

let carList = cars.map(car => {
   // code here
});

请记住map()为原始数组中的每个元素创建一个新元素。如果您只想对列表中的某些元素执行操作，则应首先使用filter()：

let carList = cars.filter(car => {
   return car.owner !== undefined
}).map(car => {
   // Do something to each car
});

请注意，undefined是一个关键字，不需要引号。使用字符串值"undefined"，虽然合法，但可能令人难以置信的混淆，所以我建议反对它。

Answer 2

听起来你需要filter而不是map：

＆＃13;

const cars = [
  { owner: '', age: 12 },
  { owner: 'bob', age: 1 },
  { owner: undefined, age: 2 },
  { owner: 'steve', age: 13 },
  { age: 1 }
];

let carList = cars.filter(car => car.owner);

console.log(carList);

＆＃13;

Answer 3

如果您想通过字符串进行比较，则需要添加typeof 另外，用大括号括起函数。

let carList = cars.map(car => {
  if(typeof(car.owner) !== "undefined"){ // this is the condition I want.
  do something
 }
});

或抛弃字符串引号

let carList = cars.map(car => {
  if(car.owner !== undefined){ // this is the condition I want.
  do something
 }
});

或者只使用布尔值而不是。

let carList = cars.map(car => {
  if(!car.owner){ // this is the condition I want.
  do something
 }
});

let cars = [{},{owner:'foo'},{owner:'bar'}, {fun:'none'}];
let carList = cars.map(car => {
  if(!car.owner){ // this is the condition I want.
    console.log(car,'has no owner');
 }}
);

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