Detailed analysis of JavaScript function definition_javascript skills

Function

A few key points:

a). Functions are first-class citizens in JavaScript (importance)
                                                                                                                                                                                                                                                                                     . c). The function defines an independent variable scope

Definition method

a) Named function:

Named functions are global unless defined inside another function.

      // 全局的命名函数
  function add(x, y) {
    return x + y;
  }
  console.info(add(100, 200));  //300

b) Anonymous function:

Anonymous functions are usually assigned to a variable and then called through the variable.

    var func = function (x, y) {
      return x + y;
    }
    console.info(func(5, 2)); //7

Anonymous functions are suitable for the following situations of “immediately executed anonymous functions”:

    console.info(
      function (x, y) {
            return x + y;
          }(100, 200)  //立即调用
        );

C) The definition method affects the code execution effect

The named function can be used first and then defined

    console.info(sum(10, 10));
    function sum(num1, num2) {
      return num1 + num2;
    }

Anonymous functions must be defined first before using

    //console.info(sumFunc(10, 10));  //Uncaught TypeError: Property 'sumFunc' of object [object Object] is not a function 
    var sumFunc = function (num1, num2) {
      return num1 + num2;
    };
    console.info(sumFunc(10, 10));

Function return value:

Use return to generate a return value. If there is no return, the function returns undefined

 function func() {
 }
 console.info(func()); //undefined
 function func2() {
   return; //空的返回语句
 }
 console.info(func2()); //undefined

The pit hidden in the return:

 var func = function (x, y) {
   var sum = x + y;
   return {
     value : sum
   }
 }

There is no problem in writing like this: calling func(5,5) returns Object {value: 10}

However:

  var func = function (x, y) {
    var sum = x + y;
    return
    {
      value: sum
    };
  }
  console.info(func(5,5)); //undefined

If return is followed by a carriage return and a line change,

Calling func(5,5) displays undefined
The editor added a semicolon after the return for us; however, it is of no use in this case.

Functions are objects:

  function add(x, y) {
    return x + y;
  }
  console.info(add(100, 200)); //300
  var other = add; //other和add引用同一函数对象
  console.info(other(300, 400)); //700
  console.info(typeof other);  //function
  console.info(add === other); //true

Nested defined functions:

Within a function, you can define another function.

  function outerFunc(a, b) {
    function innerFunc(x) {
      return x * x;
    }
    return Math.sqrt(innerFunc(a) + innerFunc(b));
  }
  console.info(outerFunc(3, 4)); //5

Access external variables:

Internal functions can access external variables and parameters.

 var globalStr = 'globalStr';
 function outerFunc2(argu) {
   var localVar = 100;
   function innerFunc2() {
     localVar++;
     console.info(argu + ":" + localVar + ":" + globalStr);
   }
   innerFunc2(); //hello:101:globalStr
 }
 outerFunc2("hello");

Function that returns a function:

Because functions are objects, they can be used as return values.

  function outerFunc(x) {
    var y = 100;
    return function innerFunc() {
      console.info(x + y);
    }
  }
  outerFunc(10)(); //110

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