The class declaration creates a new class with a given name using prototype-based inheritance.
You can also define a class using a class expression. But unlike a class expression, a class declaration doesn't allow an existing class to be declared again and will throw a SyntaxError
if attempted.
class name [extends otherName] { // class body }
The class body of a class declaration is executed in strict mode. The constructor
method is optional.
Class declarations are not hoisted (unlike function declarations).
In the following example, we first define a class named Polygon
, then extend it to create a class named Square
.
Note that super()
, used in the constructor
, can only be used in constructors, and must be called before the this
keyword can be used.
class Polygon { constructor(height, width) { this.name = 'Polygon'; this.height = height; this.width = width; } } class Square extends Polygon { constructor(length) { super(length, length); this.name = 'Square'; } }
Re-declaring a class using the class declaration throws a SyntaxError
.
class Foo {}; class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared
The same error is thrown when a class has been defined before using the class expression.
let Foo = class {}; class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared
Desktop | ||||||
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class |
49
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13 | 45 | No | 36
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10.1 |
Mobile | ||||||
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class |
49
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49
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45 | 36
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10.3 | 5.0
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Server | |
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class |
6.0.0 |
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https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class