What does “return” do in Javascript?

问题

I just taught myself how to code with tutorials from the internet. I'm currently trying to learn Javascript and I don't really undesrtand the purpose of "return". I made a "rock, paper, scissors" game at the end of my lesson, using the return function. The game looks like this:

var userChoice = prompt("Do you choose rock, paper or scissors?");
var computerChoice = Math.random();
if (computerChoice < 0.34) {
    computerChoice = "rock";
} else if(computerChoice <= 0.67) {
    computerChoice = "paper";
} else {
    computerChoice = "scissors";
} console.log("Computer: " + computerChoice);

var compare = function(choice1, choice2){
    if(choice1 === choice2){
        return "The result is a tie!";
    }
    else if(choice1 === "rock"){
        if(choice2 === "scissors"){
            return "rock wins";
        }
        else{
            return "paper wins";
        }
    }
    else if(choice1 === "paper"){
        if(choice2 === "rock"){
            return "paper wins";
        }
        else{
            return "scissors wins";
        }
    }
    else if(choice1 === "scissors"){
        if(choice2 === "paper"){
            return "scissors wins";
        }
        else{
            return "rock wins";
        }
    }
};
compare(userChoice, computerChoice);

What exactly would be the difference if I used console.log("....") instead of "return" here?

回答1:

According to W3Schools,

The return statement stops the execution of a function and returns a value from that function.

But

console.log writes into the browser console.

In other words, you can't see the output of console.log unless you open 'Developer Tools'

According to MDN (a much more reliable source),

When a return statement is called in a function, the execution of this function is stopped. If specified, a given value is returned to the function caller. If the expression is omitted, undefined is returned instead. The following return statements all break the function execution:

return;
return true;
return false;
return x;
return x + y / 3;

But

console.log() outputs a message to the Web Console.

For example,

function myFunction() {
    return Math.PI;
}

would return 3.141592653589793 when the function is called.

But using console.log instead, would not show anything, on a webpage (except in developer tools).

回答2:

In JavaScript, most things are expressions, and every expression has a value. That can be a number (5), a string ("Hello, World!"), an object ({ foo: 'bar' }) or something other, like a regex.

When you use an operator, e.g. choice1 === "rock", that's also an expression, which has a boolean value (true or false). The expression 5 + 3 has a value, too. It is 8.

When you call a function, that's also an expression, which has a value. The prompt function, for example, has the value that the user entered. It's what you get back when you call prompt(...). The value of a function call is refered to as the "return value" of that function.

When you define your own function, you can also give them a return value. And later, when you call the function, this return value is what you get back. Remember functions in the maths class in school? Things like f(x) = x * x? When you "call" f with 5, then 25 is the value of that "function call": f(5) = 5 * 5 = 25.

Now, let's define our own JavaScript function that has a return value:

function add (a, b) {
  return a + b; // make the sum of a and b the return value of "add"
}

var result = add(5, 3); // store the value of the function call
console.log(result); // should log 8

Many built-in JavaScript functions have a return value: Math.min() returns the smallest of the arguments passed to it, document.getElementById() returns the HTML element with the id that you passed to it.

For some functions, it doesn't make any sense to return the result of an action. What should be the result of console.log()? Those functions return the value undefined. When your own functions don't have a return statement, their return value is undefined.

It is very important to note that the execution of a function stops when it hits a return statement. It then immediately returns to the location in the script where the function was called:

function test () {
  console.log('Hello 1');
  return 5; // return to the caller
  console.log('Hello 2'); // will not execute
}

var result = test();
console.log(result); // 5

When you leave out the value after the return ...;, so you just say return;, then the return value will also be undefined. You can use that if you want to stop the execution without a value to return:

function divide (a, b) {
  return a / b;
}

function printQuotient (a, b) {
  if (b === 0) {
    // the divisor is zero, do nothing
    return;
  }
  
  // divisor is not zero, print the result of the division
  console.log(divide(a, b));
}

printQuotient(10, 5); // prints 2
printQuotient(10, 0); // prints nothing

回答3:

The concept of return is to return something.

for ex you could have var winner = compare(userChoice, computerChoice); That would set the winner as the string returned from the function compare(), and you could use that var later for display. So in all actuality winner could be equal to any of your if/else strings. so for another ex, winner could be read as winner = 'rock wins' (if that was the logical winner)

回答4:

console.log will display or emit the parameter passed to the log method in the console window of your browser and can be used almost anywhere in the code.

Meanwhile when using the return statement, the function will stop executing, and return the specified value.

回答5:

When a return is encountered in a function. It will return to the point immediately after the code that called the function. If nothing is specified after the return statement, that is all that happens. However if a literal is returned like 2 or 3,etc, a literal is returned. You can also return a Boolean "true/false ", a string, or a variable. Console.log () means what the name implies, it literally displays the value inside the parentheses to the development console. Its primary use is for troubleshooting.

来源：https://stackoverflow.com/questions/44474975/what-does-return-do-in-javascript

标签

javascript

return

console.log