Detailed explanation of JavaScript function return values ​​and return statements

The JavaScript function provides two interfaces to interact with the outside world. The parameters serve as the entrance to receive external information; the return value serves as the exit to feed back the operation results to the outside world. The following article will take you to understand the JavaScript function return value and briefly analyze the usage of the return statement. I hope it will be helpful to you!

A function is a collection of functions that can implement certain specific functions or operations. It makes no sense to keep the result after the function is run inside the function, so we need to return the result of the function to the place where the function is called.

In the function body, use the return statement to set the return value of the function; the return statement is usually defined at the end of the function. Grammar format:

return 返回值;

Among them, "return value" is an optional parameter, which can be a specific value or expression, or it can be empty. The "return value" and the return keyword need to be separated by a space.

JavaScript is a weakly typed language, so functions have no type restrictions on the values ​​they receive and output, and JavaScript does not automatically detect the types of input and output values. Therefore, the return value (expression result) can be of any type, such as array, object, string, etc.

• For functions that have a return value, we can use a variable to receive the return value of this function. The sample code is as follows:

//声明一个带返回值的函数
function getSum(num1, num2){
        //函数体
    return num1 + num2;
}
//可以通过变量来接收这个返回值
var sum1 = getSum(7, 12);      // 函数返回值为：19
var sum2 = getSum(-5, 33);     // 函数返回值为：28

• If the function does not contain a return statement, the undefined value will be returned after each statement in the function body is executed.

• In the function, once the "return" statement is executed, the entire function ends. The statement will no longer be executed ; then the value of the expression following return is evaluated and returned.
  

• The value after "return" can only have one . If you try to return multiple values, the result will always be the last value
  

return statement can only return one parameter , that is, only one value can be returned, and multiple values ​​cannot be returned at one time. If you want to return multiple values, you need to define an array in the function, store the return value in the array and return it.

function division(dividend, divisor){
    var quotient = dividend / divisor;
    var arr = [dividend, divisor, quotient]
    return arr;
}
var res = division(100, 4)
console.log(res[0]);         // 输出：100
console.log(res[1]);         // 输出：4
console.log(res[2]);         // 输出：25

In the above code, the function return value is an array, which contains 3 elements, thus achieving the purpose of using a return statement to return multiple values.

• The function body can contain multiple return statements, but only one return statement can be executed, so the branch structure can be used in the function body to determine the function return value, or use the return statement to terminate function execution early.

function f(x,y) {
    //如果参数为非数字类型，则终止函数执行
    if (typeof x != "number" || typeof y != "number") return;
    //根据条件返回值
    if (x > y) return x - y;
    if (x < y) return y - x;
    if (x * y <= 0) return x + y;
}
console.log(f(1,6));
console.log(f(4,2));
console.log(f(0,0));

Summary:

• The return statement is used to call a function "or" returns a value. After returning the value, the function execution ends immediately, so the return statement is generally placed at the end of the function;

• If there are multiple return statements in a function, only The first one will be executed;

• The return statement may not return parameters, which is equivalent to ending the function running;

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