如何评估此if语句

Question

var wait = function() {
   return setTimeout(function() {
      return 8;
   }, 1000);
}

var foo = function() {
   if (wait() === 8) {
      return 99;
   } else {
      return 23;
   }
}

console.log(foo());

打印23

我理解函数调用是异步的; 但是,if wait()函数如何在函数返回之前进行求值？我试图实现的逻辑如何在javascript中成功表示？

Answer 1

我理解函数调用是异步的;

函数调用完全同步.

在wait()函数返回之前,如何评估if块？

您的wait函数会立即返回setTimeout的结果,即Timeout ID.传递给setTimeout的函数将在以后的某个时间执行,返回值将丢失.

我试图实现的逻辑如何在javascript中成功表示？

您可能正在寻找的代码是这样的.我已将您的代码手动转换为Continuation Passing Style

// `wait` takes a callback which is captured in it's closure to be used by
// the anonymous function passed to setTimeout, sometime in the future.
var wait = function(callback) {
   return setTimeout(function() {
      callback( 8 );
   }, 1000);
}

// `foo` also takes a callback that will be called when the function passed
// to `wait` is evaluated.
var foo = function(callback) {
   wait(function(value){
       if (value === 8) {
          callback( 99 );
       } else {
          callback( 23 );
       }
    }
}

// Finally, `foo` is called with another callback, this time logging the value.
foo(function(value) {
    console.log(value);
})