Understanding how JavaScript Prototypes work - Stack Overflow

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Am messing around with prototypes to get a better understanding of how they work. I can't work out why I can't call hideHeader, whereas I can access a variable (this.header.el)

function App() {
    this.init();
    this.el = document.getElementById('box');
}

App.prototype.init = function () {
    document.write('hello world');

    this.header = new Header();

    this.header.hideHeader();
    this.header.el.style.display = 'none';
};

new App();

function Header() {
    this.el = document.getElementById('header');
}

Header.prototype.hideHeader = function() {
    this.el.style.display = 'none';
}

Am messing around with prototypes to get a better understanding of how they work. I can't work out why I can't call hideHeader, whereas I can access a variable (this.header.el)

function App() {
    this.init();
    this.el = document.getElementById('box');
}

App.prototype.init = function () {
    document.write('hello world');

    this.header = new Header();

    this.header.hideHeader();
    this.header.el.style.display = 'none';
};

new App();

function Header() {
    this.el = document.getElementById('header');
}

Header.prototype.hideHeader = function() {
    this.el.style.display = 'none';
}

• javascript
• prototype

Share Improve this question Follow edited Mar 21, 2018 at 5:16 Glenn Ferrie 10.4k33 gold badges4444 silver badges7676 bronze badges asked May 17, 2015 at 19:51 AlexAlex 17711 silver badge77 bronze badges 1

• 2 Observe what happens when you move the call to App() at the bottom, and also note that document.write overwrites the document – adeneo Commented May 17, 2015 at 19:57

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2 Answers 2

Sorted by: Reset to default 9

You should reorder your code so that you define hideHeader before you try to invoke it.

Like this:

function App() {
    this.init();
    this.el = document.getElementById('box');
}

function Header() {
    this.el = document.getElementById('header');
}

Header.prototype.hideHeader = function() {
    this.el.style.display = 'none';
}

App.prototype.init = function () {
    document.write('hello world');

    this.header = new Header();

    this.header.hideHeader();
    this.header.el.style.display = 'none';
};

new App();

JavaScript is an interpreted language, it's not piled. It is evaluated sequentially as it is loaded into memory.

You just need to change the order of how you are doing things. For example:

function App() {
    this.init();
    this.el = document.getElementById('box');
}


function Header() {
    this.el = document.getElementById('header');
}

Header.prototype.hideHeader = function() {
    this.el.style.display = 'none';
}

App.prototype.init = function () {
    document.write('hello world');

    this.header = new Header();

    this.header.hideHeader();
    this.header.el.style.display = 'none';
};

new App();

<div id="header"></div>
<div id="box"></div>

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