Question

someOperation.then(function(x) {
    things.forEach(function(thing) {
        //doing something with 'thing' that depends on variable 'x'
    });
});

在上面的代码中，如何在回调函数中使变量'x'可用？或者在这种情况下我是否必须回到使用for循环？

Answer 1

您可以将“thisArg”作为第二个参数传递给forEach，例如：

let x = { a: 123 };
things = ['foo', 'bar']
things.forEach(function(thing) {
    alert( this.a + thing );
}, x);

根据您的目的，可能会有所帮助。

Answer 2

可用。

let x = {
  name: 'Mike'
};
['Hello', 'Goodbye'].forEach(function(greeting) {
  document.querySelector('pre').innerHTML += greeting + ', ' + x.name + '\n';
});

<pre></pre>

您在此处使用的内容称为closure，是Javascript的常用功能。基本上，任何函数都可以访问其父作用域中的任何其他变量。

function log(msg) {
  document.querySelector('pre').innerHTML += msg + '\n';
}

var global = 'global';
function firstLevel(third) {
  var first = 'first';
  
  // `global` is in the parent scope so we can access it
  log(global + ' -> ' + first);
  
  function secondLevel() {
    var second = 'second';
    
    // Same thing with `first` here
    log(global + ' -> ' + first + ' -> ' + second);
    
    // This even works with passed in arguments
    log(global + ' -> ' + first + ' -> ' + second + ' -> ' + third);
    
    // We can even change closed over variables
    first = 'fourth?';
  }
  
  secondLevel();
  log(first); // Notice that `first` changed.
}

log(global);
firstLevel('third'); // Notice how `third` is used in `secondLevel`

<pre></pre>

将参数传递给Array.forEach回调函数

2 个答案: