A summary of how named functions are called in JavaScript

It was mentioned in a previous post (link: #). This article looks at the various ways in which named functions can be called.

1, ()  

The most commonly used is the () operator to call/execute a function:

//The function fun1

function fun1() {

    alert(' I got called ');

}

fun1();

 

//The argument function fun2

function fun2(param) {

    alert(param);

}

fun2(' I got called ');

After ECMAScript3 is added, call and apply are added to Function.

2, call

//The function fun1

function fun1() {

    alert(' I got called ');

}

fun1.call(null);

 

//The argument function fun2

function fun2(param) {

    alert(param);

}

fun2.call(null,' I got called ')

3, apply

//The function fun1

function fun1() {

    alert(' I got called ');

}

fun1.apply(null);

 

//The argument function fun2

function fun2(param) {

    alert(param);

}

fun2.apply(null,[' I got called '])

Although call and apply can be used purely to call/execute functions, they are more used to change the context in which the function is executed.

4. New (not recommended)

//The function fun1

function fun1() {

    alert(' I got called ');

}

new fun1();

 

//The argument function fun2

function fun2(param) {

    alert(param);

}

new fun2(' I got called ')

The essence of new is to create/construct an instance of a class, where fun1 and fun2 are clearly not a class (no this, no prototype). But two functions do execute. This is a side effect of new.

There is no difference between the four methods in terms of the above method. But if the function returns a value, you may be disappointed when called in new.

//The function fun
 with a return value
function fun() {

    alert(' I got called ');

    return "jack";

}

var c = new fun();

alert(c);//[object object], why not "jack"? < br / >

To such

//The function fun
 with a return value
function fun() {

    alert(' I got called ');

    return {name:'jack'};

}

var c = new fun();

alert(c.name); //Jack, returns < br / > as normal

To summarize: when calling a function in new mode. If a return value exists, it will not be returned when the return value is a JavaScript built-in type (primitive type) such as String, Number, Boolean, etc. When the return value is an object, a function, an array, etc., it returns the object, function, array, etc.

What exactly does new fun() return when the return value is a built-in (primitive) type? The next article will discuss the details of new mode invocation.