CODEWITHSUNDEEP
javascriptgetterobject-literal
Boris
Boris

Reputation: 166

What does it do when a method definition inside an object literal is prefixed by the `get` keyword?

I'm using the library handsontable and I'd like to get my application running in IE8-IE9 (even though it's not IE < 10 compatible...). When my code use the minify version I get an error in the JS console : "';' expected".

Here is the code.

{
    get DEFAULT_WIDTH() {
        return 50;
    }
}

I just don't know this syntax. What does get DEFAULT_WIDTH() do?

Upvotes: 1

Views: 231

Answers (2)

connexo
connexo

Reputation: 56803

The get syntax binds an object property to a function that will be called when that property is looked up.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get

The more general and exhausting explanation can be found here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_Objects#Defining_getters_and_setters

Basically, it allows you to define what happens when a certain object property is read by code. In an analogous fashion, you can also define what should happen when code writes to that property with a set definition. In both cases you overwrite the standard behaviour for that object property.

This is all part of ECMAScript 5.1, and thus, not available in IE < 9.

What does your example code do?

In your example code, you can see that whenever the property DEFAULT_WIDTH is read, a constant value will be returned. I guess the intention of this is to make sure DEFAULT_WIDTH cannot be redefined as some other value (which it in fact can, but reading it will still return 50).

Defining a getter on existing objects using defineProperty

To append a getter to an existing object later at any time, use Object.defineProperty().

var o = { a:0 }
Object.defineProperty(o, "b", { get: function () { return this.a + 1; } });
console.log(o.b) // Runs the getter, which yields a + 1 (which is 1)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get#Defining_a_getter_on_existing_objects_using_defineProperty

Upvotes: 0

Simon Groenewolt
Simon Groenewolt

Reputation: 10665

MDN has documentation for get, including a list of supporting browsers. What get does is invoke a function when the property is looked up. See Defining getters and setters for a more general explanation.

Upvotes: 1

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