When is return required and when return is not required in a JS function

In JavaScript functions, you need to use the return keyword to return the result of function execution, which usually happens in the following situations:

1. Return a value: If a function needs to calculate and return a value, it needs to use the return keyword to return the value to the caller. For example:

function addNumbers(a, b) {
  return a + b;
}

In this example, the addNumbers function receives two parameters and returns their sum, so return is needed to return the calculated result.
2. Early exit function: If the function needs to exit early when some conditions are not met, you can use return to exit the function and return a specific value or undefined. For example:

function validateInput(input) {
  if (input === '') {
    return '输入不能为空';
  }
  if (input.length < 5) {
    return '输入至少需要5个字符';
  }
  // 如果输入有效，则不返回任何值
}

In this example, the validateInput function takes an input value and validates against the length of the input value. If the input value is empty or the length is less than 5, use the return keyword to return a specific error message. Returns no value if the input value is valid.

Other than the above, JavaScript functions generally do not need to use the return keyword. If the function does not return a value, it will return undefined by default. For example:

function greet(name) {
  console.log('Hello, ' + name + '!');
}

var result = greet('John'); // 输出：Hello, John!
console.log(result); // 输出：undefined

In this example, the greet function outputs a greeting message, but returns no value. Therefore, when we try to store its result in the variable result, the value of the variable is undefined.

In JavaScript, the console.log method is used to print a message to the console, but it does not return any value. Therefore, if you need to print results back from a function, you should use the return keyword instead of console.log.

For example, suppose we have a function sum that calculates the sum of two numbers and prints the result. If we need to return calculation results from a function, we should use the return keyword instead of console.log. For example:

function sum(a, b) {
  var result = a + b;
  console.log(result); // 在控制台打印结果
  return result; // 返回计算结果
}

var total = sum(3, 4); // 调用 sum 函数，并将结果存储在变量 total 中
console.log(total); // 在控制台打印结果

In this example, the sum function calculates the sum of two numbers and stores the result in the variable result. Then, the function uses console.log to print the calculation result on the console, and uses the return keyword to return the result to the caller. When calling the function, we can store the calculation result in the variable total and print the result on the console.

It should be noted that if the main purpose of the function is to print a message rather than return a value, there is no need to use the return keyword. In this case, use the console.log method to output the message.