JS function definition and invocation

Since the function of JavaScript is also an object, the abs() function defined above is actually a function object, and the functions name abs can be considered as a variable pointing to the function.

var function (x) {    if (x >= 0) {        return  x;     Else {        return -x;    }};

这种方式下，function (x) { ... }是一个匿名函数，它没有函数名。但是，这个匿名函数赋值给了变量abs，所以，通过变量abs就可以调用该函数。

abs()//返回NaN

abs(x)the parameters of the function are received at this time x undefined , and the computed result is NaN .

To avoid receipt undefined , you can check the parameters:

function ABS (x) {    if (typeof x!== ' number ') {        throw ' not a number '; 
    }    if (x >= 0) {        return  x;     Else {        return -x;    }}

Arguments

JavaScript also has a free keyword arguments that works only inside the function and always points to all parameters passed in by the caller of the current function. argumentssimilar Array but it is not a Array :

function foo (x) {    //  tenfor     (var i=0; i<arguments.length; i++) { c10/>//  ten,    }}foo (10, 20, 30);

Using arguments , you can get all the parameters that the caller passed in. That is, even if the function does not define any parameters, it can get the value of the parameter:

function abs () {    if (arguments.length = = = 0) {        return 0;    }     var x = arguments[0];     return x >= 0? X:-/  /0//   // 9

In fact arguments , it is most commonly used to determine the number of incoming parameters. You may see the following notation:

// foo (a[, b], c) // receives two or three parameters, B is an optional parameter, and if only 2 parameters are passed, B defaults to null: function Foo (A, B, c) {    if (arguments.length = = = 2) {        //  The actual parameters obtained are A and B, C for undefined        //  assign B to c        null//  B to default value     }     //  ...}

Rest parameters

The ES6 standard introduces rest parameters

function Foo (A, B, ... rest) {    Console.log (' a = ' + a);    Console.log (' b = ' + b);    Console.log (rest);} Foo (1, 2, 3, 4, 5);

JS function definition and invocation