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程序执行完成后返回values。
You've already met return values a number of times, although you may not have thought about them explicitly.

let myText = 'The weather is cold';
let newString = myText.replace('cold', 'warm');
console.log(newString); // Should print "The weather is warm"

// the replace() string function takes a string,
// replaces one substring with another, 
// and returns a new string with the replacement made

The replace() 函数is调用invoked on the myText string, and is passed two parameters, 传递2个参数：
1. the substring to find ('cold').
2. the string to replace it with ('warm').

一些函数不返回任何值。

function draw() {
  ctx.clearRect(0, 0, WIDTH, HEIGHT);
  for(let i = 0; i < 100; i++){
    ctx.beginPath();
    ctx.fillStyle = 'rgba(255,0,0,0,0.5)';
    ctx.arc(random(WIDTH), random(HEIGHT), random(50), 0, 2 * Math.PI);
    ctx.fill();
  }
}

Insides each loop iteration, three calls are made to the random() function, to generate random value for the current circle's x-coordinate, y-coordinate, and radius, respectively. The random() function takes one parameter -- a whole number -- and it returns a whole random number between 0 and that number. it looks like this:

This return value appears at the point the function was called, and the code continues.

input.onchange = function() {
  const num = parseFloat(input.value);
  if (isNaN(num)) {
    para.textContent = 'You need to enter a number!';
  } else {
    para.textContent = num + ' squared is ' + squared(num) + '. ' +
                       num + ' cubed is ' + cubed(num) + '. ' +
                       num + ' factorial is ' + factorial(num) + '.';
  }
}

function mySandwich(param1, param2, callback) {
  console.log('Started eating my sandwich.\n\nIt has: ' + param1 + ', ' + param2);

  $('#sandwich').animate({
    opacity: 0
  }, 5000, function() {
    console.log('Animation complete!');
  });

  if (callback && typeof(callback) === "function") {
    callback();
  }
}

mySandwich('ham', 'cheese', function() { 
  console.log('Finished eating my sandwich.');
});

(callback && typeof(callback) === "function") && callback();