如何从类中的另一个函数调用函数

Question

I have the following problem: I want to call the function printHello from the function testHello . The printHello function works on its own, however, when I try to call printHello from the testHello function, I get a reference error. Thank you for your help.

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 class Test { constructor(name) { this.name; } printHello(parameter) { console.log(parameter); } testHello() { printHello(printHello(this.name)); } } var test = new Test("Sandro"); test.printHello("hello"); //works, prints "Hello" to the Console test.testHello(); // does not work: Reference Error: printHello is not defined

Answer 1

Use this keyword. Also, you had a few bugs (i commented them)

 class Test{ constructor(name){ this.name = name; // <- you need to assign the `name` to `this.name` } printHello(parameter){ console.log(parameter); } testHello(){ this.printHello(this.name); // <- you had double invocation here } } var test = new Test("Sandro"); test.printHello("hello"); //works, prints "Hello" to the Console test.testHello(); // does not work: Reference Error: printHello is not defined

Answer 2

You probably need to call

this.printHello(this.name);

inside your testHello Function.

Answer 3

A couple of issue in the code that can be easily fixed.

1) You need to set the this.name property in the constructor.

2) You want to prefix the call to printHello within testHello with this.

 class Test { constructor(name) { //You need to actually set this.name to the parameter value this.name = name; } printHello(parameter) { console.log(parameter); } testHello() { //this keyword is needed to prefix the method name this.printHello(this.name); } } //Now below should work with no issues var test = new Test("Sandro"); test.printHello("hello"); test.testHello();

Answer 4

 class Test { constructor(name) { this.name = name; } printHello(parameter) { console.log(parameter); } testHello() { this.printHello(this.name); } } var test = new Test("Sandro"); test.printHello("hello"); //works, prints "Hello" to the Console test.testHello(); // does not work: Reference Error: printHello is not defined

var test is defined on the global level. This returns an object that contains name property. In the prototypes, we can see constructor, printHello and testHello functions. Now when you call //test.printHello// it will work without a problem. Why? when the function is not at the same level, then it will go down to prototypes until it meets the function. In the prototype, you can see the printHello function. This is call prototype inheritance.

Now what happens in //test.testHello()// . javaScript trying to execute this one and see another function inside that. So it will try to find where this printHello function defined!.

Now we need to know about the lexical environment. Check this sample,

    var test = 'hello' ; 
    function foo(){
       var test = 'world'
       console.log(test)
    }

Now what will happen if you call foo()? it will console.log 'world',

Now I'm removing test var inside the function

    var test = 'hello' ; 
    function foo(){
       console.log(test)
    }

Now let's call foo(); The output will be 'hello'

1. When the function executes, javascript trying to find the test variable. Oh, it's on the same level. In other word functional level. Now javaScript know the value of test so it will execute console.log();
2. This time javascript try to find test variable. mmm, it's not inside the same level. (functional level). Ok, no problem. Let see where is this function defined. Oh, it's defined on the global level. Ok, now I'm gonna find this test var at the global level. ah ha, here it is. its value is 'hello'

The same thing happens with your code. Once javascript trying to execute test.testHello(), it sees printHello() inside that function. Now it will try to find the function at the same level. It is not defined in the testHello() function neither inside prototypes of testHello(). So now javaScript trying to find out where is this testHello() function defined. It is on the global level because var test is defined on the global level. Is this printHello() defined at the global level? NO! It is inside the prototype of that object. Now javaScript cannot find the printHello function in global level either. So it will throw an error that saying printHello() is undefined. 'this' keyword help you to refers to the object it belongs to. Simply, with 'this' keyword we are asking please check the test object's prototype before search on the global level. (notice that not in the testHello() prototypes but in the test object prototype)

Something important to understand is functions are not actually functions. They are objects in javaScript!!! Use 'this' keyword. It will help. Also, try to learn about prototype inheritance and lexical environment.